BIO112 Laboratory Guide #5
DIVERSITY OF THE INVERTEBRATES I
INTRODUCTION
The invertebrates are an extremely large and diverse group of organisms. They include animals that are of interest from economic and biomedical viewpoints as well as organisms that are just fascinating and of natural appeal. The diversity of the invertebrates may be organized into from 8 to over 30 extant phyla, most with numerous classes, orders, etc. It is important to keep both the taxonomic placement and the underlying phylogeny of the organisms under study in mind as you go through the exercises. In this way you will begin to learn the relationships among groups as you explore their morphology, anatomy, and life histories.
There is an essential between the evolution of animals and that of plants. As you will see in lab #9, plant evolution is closely tied to the colonization of land and the slow radiation of progressively "higher" plants into ever drier and more inhospitable niches. It is characterized by the successive replacement of the nonvascular mosses first by the vascular ferns, then by seed-bearing gymnosperms, then finally by angiosperms as the dominant and most speciose group. In sharp contrast all of today's animal phyla (along with even more extinct ones) appeared in the oceans during a very short evolutionary burst some 650 million years ago, during the "Cambrian Explosion". Each phylum has radiated and diversified to a greater or lesser degree over the course of time, with most eventually becoming extinct. The invasion of the land allowed some of these phyla to diversify enormously, but no completely new body plans or life cycles were developed. At present, the mollusks, chordates, and especially arthropods seem to be enjoying the most success. One way of graphically visualizing this difference is to picture the evolutionary "tree" of the plant divisions as a climbing vine and that of the animal phyla as a very broad-based shrub.
A note on the grouping of organisms in the class vs. the lab is in order here. Your text presents the invertebrates in an order corresponding to the established phylogenetic taxa, based on molecular sequencing and some finer aspects of shared derived embryology. In contrast, the lab exercises will group the invertebrates for study on the basis an older taxonomy based on body plans using the terminology listed here:
Diploblastic: being derived from two primordial germ cell layers, endoderm and ectoderm. (True mesoderm is missing in these organisms)
Triploblastic: being derived from three primordial germ cell layers, endoderm, ectoderm, and mesoderm
Acoelomate: lacking a true body cavity; this term is typically restricted just to the triploblastic organisms
Pseudocoelomate: having a body cavity which is not lined with mesoderm; the coelom lies between the mesoderm and the endoderm
Coelomate: having a body cavity lined with mesoderm. The body cavity may be partitioned with septa, either bilaterally or segmentally
Schizocoelous : having a true body cavity that forms from the splitting of solid blocks of mesoderm to open a new (novel) cavity
Enterocoelous: having a true body cavity that forms from the outpocketing of a portion of the preexisting archenteron (primitive gut cavity)Protostome: having the mouth or oral opening develop from the blastopore and the anus develop as a secondary opening from the gut; the mouth is literally the "first opening"
Deuterostome : having the anus or aboral opening develop from the blastopore and the mouth develop as a secondary opening into the gut; the mouth is literally the "second opening"
In most cases these alternative taxonomies line up. A notable pair of exceptions will be the nematodes and arthropods. Traditional taxonomy places the nematodes with the rotifers as pseudocoelomates and the arthropods with the molluscs and annelids as schizocoelomates. The more modern cladistic taxonomy places the nematodes and arthropods together in the separate clade Ecdysozoa, based on the developmental use of the hormone ecdysone.
In the first part of this lab exercise we will explore the defining forms and systematics of some of the large phyla of parazoans (Ph. Porifera), diploblastic radiates (Ph. Cnidaria & Ctenophora), acoelomates (Ph. Platyhelminthes), pseudocoelomates (Ph. Rotifera & Nematoda), and the protostomic schizocoelomates (Ph. Mollusca, Annelida, & Arthropoda). We will explore the remaining major group, the deuterostomic enterocoelomates in Lab 6.
Many invertebrate taxa go through characteristic developmental stages on their way from zygote to adult. These sequential "life histories" or "life cycles" can be remarkably complex, involve stages with very different structures and ecological niches, and involve sophisticated physiological and behavioral adaptations. In the second part of this lab we will take a close look at three of these life cycles and how they relate to both the reproduction and the ecology of the organisms involved
There are two general means of reproduction among the animals, asexual reproduction and sexual reproduction. Asexual reproduction normally involves only one parent, only diploid mitosis (not meiosis), and no special organs or cells. The result is a new individual that is genetically identical to its parent, essentially a clone. Methods of asexual reproduction include fission (splitting) and budding. Asexual reproduction in animals is found primarily among the relatively simpler animal forms such as poriferans, cnidarians, and platyhelminths. Sexual reproduction is found in a wide variety of animals and some organisms can reproduce both sexually and asexually. Nearly all vertebrates and many invertebrates have separate sexes, i. e. males and females are separate organisms. Such organisms are called dioecious. In some cases both sexes are present in one individual. Such organisms are called monoecious or hermaphroditic. Some animal taxa exhibit both asexual and sexual reproductive phases or stages during their life cycle.
One a zygote has been formed by fusion of the egg and sperm (syngamy), sexually reproducing animals animals may exhibit relatively direct or indirect development. Direct development means that the developing zygote moves through a steady pattern of morphological changes that leads directly to the attainment of the basic morphology of the adult organism. These intermediate forms which resemble adults but are not sexually matures are often called nymphs or juveniles. In comparison, animals that exhibit indirect development proceed through one or more distinct larval stages. A larva (and/or pupa) is an early, sexually immature developmental stage of an animal that is distinctly dissimilar in basic morphology from the adult of that species. Many times these larval stages will undergo metamorphosis, a rapid change in morphology, as they pass from one larval stage to the next or from the final larval stage to the reproductively mature adult.
After completing this laboratory you should be able to:
1) outline and describe in appropriate terms the diversity of the invertebrate animals;
2) describe the basic body plans of the major invertebrate phyla;
3) group these phyla based on central features of the body plan and developmental patterns using both traditional and modern cladistic systematics;
4) describe the major patterns of variation and taxonomic sub-groupings within these major phylum - down to at least the level of taxonomic Class;
5) identify and classify representative animals from each phylum and class;
6) describe the unique ecological features of each phylum and class;
7) identify the reproductive means and methods for the major invertebrate groups
8) trace the life cycles and major developmental stages of several representative invertebrates.
PART I. DIVERSITY OF THE INVERTEBRATES
Materials
Live, preserved, dried, and/or fossil specimens for each taxonomic group.
Poster guides to the diversity within each phylum.
Procedure
Specimens of each major invertebrate phylum and many major classes are on display. These specimens include preserved animals, concreted structures such as shells, and live animals.
1. Work through the displays of preserved specimens and microscopic slides.
a. A dichotomous key which outlines the major distinguishing features of each invertebrate group/class is provided below.
b. Following the key is additional information about each phylum, as well as a guide to lower taxa (subphylum, class) within each phylum. On the sample specimens, identify and examine all of the external structures written in bold print in this guide.
c. Additional information will be on display with the specimens.
d. Your textbook has additional information on these invertebrate phyla. The Biology Department has several invertebrate biology textbooks.
2. Your instructor has assembled a variety of live specimens. Compare these live specimens with the preserved ones. Handle the live specimens with extreme care! Accompanying each live specimen will be additional information and instructions. Read through and follow these to gain additional insight into the structure, physiology, and behavior of theses animals.
3. Be able to describe the distinguishing features of each phylum of invertebrates. Within each phylum, be able to recognize the major subdivisions.
4. Be able to use the distinguishing features to reliably classify these sample invertebrates.
Study Suggestions
1. Make detailed sketches and notes on specimens. This will help you to look at the specimens more closely, as well as to help you study later.
2. Plan to come view the specimens once or twice more before the lab test. Test yourself by attempting to identify the specimens as accurately as possible by their common names, as well as to classify them without first looking at their labels.
3. The words in bold print in the extended guide below are words you should know and/or structures you should be able to identify or describe.
A Dichotomous Key to the Adult Invertebrates
Note: This key is based, in some cases, on secondarily derived characteristics of adult animals. Therefore, it does NOT always follow phylogenetic relationships. It might be useful for you to construct an alternative key which does reflect actually phylogeny, both to help you understand the true systematics of the invertebrates, and to see why a pragmatic, balanced key might not follow these.
1. asymmetric, radial, or seconadarily pentaradial body
2. asymmetrical body; sessile, filter-feeding adult
3. lacking true tissues or nervous system Ph. PORIFERA
3. coelomate; pharynx with gill slitsPh. CHORDATA; SubPh. Urochordata [lab #6]
2. radially or pentaradially symmetric body
3. radial symmetry, bag-like body; diploblastic
4. stinging cells on tentacles Ph. CNIDARIA
4. ciliated combs, adhesive cells on tentacles Ph. CTENOPHORA
3. pentaradial body; triploblastic; enterocoelomatePh. ECHINODERMA [lab #6]
1. bilaterally symmetrical body; may have radially arranged limbs
2. unsegmented body
3. acoelomate, body dorso-ventrally flattened, blind digestive tract
Ph. PLATYHELMINTHES; Cls. Turbellaria , Trematoda
3. pseudocoelomate or coelomate; two-ended digestive tract
4. pseudocoelomate
5. body elongated and worm-like Ph. NEMATODA
5. microscopic; mouth bearing ciliated wheel-organs Ph. ROTIFERA
4. coelomate
5. dorso-ventrally paired shells; enterocoelomatePh. BRACHIOPODA
5. shell single, laterally paired, internal or absent; schizocoelomate;may show visceral torsion or radial limbs Ph. MOLLUSCA
2. internally and/or externally segmented body; some segments may be fused
3. acoelomate; body dorso-ventrally flattened; no digestive system
Ph. PLATYHELMINTHES; Cl. Cestoidea
3. schizocoelomate or enterocoelomate
4. schizocoelomate
5. open circulation
6. jointed appendages Ph. ARTHROPODA
6. non-jointed appendages Ph. ONYCHOPHORA
5. closed circulation Ph. ANNELIDA
4. enterocoelomate [Lab #6]
5. dorsal notochord Ph CHORDATA; Subph. Cephalochordata
5. no notochord; prominent everted proboscisPh. HEMICHORDATA
GROUP I. PARAZOA
Members of Subkingdom Parazoa lack both the true tissue-level organization and the nervous system of the other animals. Sponges are the only extant parazoans.
PHYLUM PORIFERA (SPONGES)
The sponges, aptly named as the Phylum Porifera, or "pore-bearers", are poorly organized ablastic (tissue-less) animals that are in some ways little more than colonies of cells. Sponges are highly dependent upon a few limited types of cells, each working somewhat independently, yet still contributing to the overall good of the sponge as a whole, hence the colonial nature of this simple organism. Sponges function by using specialized flagellated cells (choanocytes) to create a water current that passes through the wall, bringing in essential materials from the surrounding water and carrying away waste products. The most common error in understanding sponge design is to interpret the large opening at the top of most sponges as a "mouth". This opening, called the osculum, is actually the exit of the water current, not the entry. The correct path of water flow is into the sponge through the typically microscopic pores along the walls and out through the osculum. We will examine three classes within this phylum.
Class Calcarea
The sponges in this group have spicules composed of calcium carbonate (CaCO3). They are typically very small in size and may appear as a thick fuzzy layer encrusting undersea rocks, shells, or coral.
Examine the prepared slides of Leucosolenia, both longitudinal and cross sections. Note the simple structure of this sponge. Note the spicules, the skeletal elements of sponges. They should appear as glassy, 3-rayed stars.
Examine the prepared slides of Scypha (=Grantia), a more complex sponge. Note that the walls are convoluted (folded).
Class Demospongia
This class is by far the largest of all of the classes of sponges and includes the most complex types of sponges. Most larger marine sponges, and all freshwater sponges belong to this class. The demosponges have a skeletal structure composed of spicules made of silica and/or spongin, a soft, proteinaceous material that forms a spongy matrix. Sponges with a skeleton made only of spongin are called the “bath sponges”.
Examine the specimens of sponges of the class Demospongia on display. Locate the major features of the typical sponge morphology for each.
Class Hexactinellida
The name of this class is derived from the fact that the spicules typically have six points. Some of the spicules fuse to form a lattice-like siliceous skeleton, and therefore are also called glass sponges. Examine the representative of this class on demonstration, commonly called the “Venus’s flower basket” .
GROUP II: EUMETAZOA (or METAZOA)
True metazoans have cells organized into specialized tissues and have nervous systems for coordinating both physiology and behavior. A major taxonomic distinction is between the protostomes, in which the blastopore becomes the mouth and the deuterostomes, in which the blastopore becomes the anus. The first several groups of phyla which we will be studying are all protostomic.
GROUP IIA. RADIATA
These phyla have radially symmetrical, bag-like, diploblastic bodies and simple neural networks.
PHYLUM CNIDARIA (STINGING-THREAD ANIMALS)
The Phylum Cnidaria derives its name from the everting stinging cells called cnidocytes, which shoot out miniature barbed harpoons called nematocysts The group has achieved a greater level of structural complexity than that of the sponges; true tissues are present and a rudimentary organ-level organization can be seen. However, these animals are diploblastic, meaning they lack the middle germ layer mesoderm. The three classes that we will examine today in lab differ from each other in several ways, including life history patterns.
As a phylum, the group displays a life cycle with metagenesis, sometimes also referred to as alternation of generations. Unlike plants, both generations are diploid - the alternation is between asexually-reproducing polyp and sexually-reproducing medusa (jellyfish) forms.
Class Medusozoa:Hydrozoa (hydroids and fire-corals)
The hydrozoans are cnidarians with simple gastrovascular cavities and have a metagenetic life cycle wherein the polyp phase is dominant. The marine hydroid Obelia is a typical representative. The more familiar fresh-water Hydra is atypical in that it totally lacks a sexually-reproducing medusa in its life cycle.
Observe prepared whole mounts of Obelia, a polymorphic, colonial hydrozoan. Locate the gastrozooids, the feeding polyps, and the gonozooids, the reproductive polyps. Locate the shared gastrovascular cavity and the perisarc, a clear covering secreted by the epidermis.
Examine the live specimens (if available) and prepared slides of the monomorphic hydroid Hydra that show the development of a bud, an asexually reproduced clone of the adult organism.
Observe the preserved specimen of Physalia, the Portuguese “Man–o–War.” Physalia is another colonial hydrozoan. However, it is atypical for hydrozoans in that it has no sessile polypoid stage. Locate the long feeding tentacles, short reproductive tentacles and the pneumatophore (float) which supports the colony at the surface of the water.
Class Medusozoa:Scyphozoa (true jellyfish)
Scyphozoans are cnidarians with a subdivided gastrovascular cavity and a metagenetic life cycle wherein the medusoid phase is dominant.
Observe the preserved specimen of the jellyfish. Locate the mouth, oral tentacles, and umbrella (bell). Note the extremely thickened mesoglea, the so-called jelly. Locate the gastrovascular cavity near the top of the umbrella. The fluffy structures extending into the cavity are the gonads. Jellyfish can be either monoecious or dioecious.
Class Medusozoa:Cubozoa (box jellyfish)
As the name somewhat unimaginatively suggests, cubozoans have box-shaped medusa as the dominant life stage. Cubozoans include some of the most venomous jellies, including the infamous sea wasp Chironex fleckeri, native to the waters around Australia.
Class Anthozoa (sea anemones and corals)
Anthozoans are cnidarians with a complex gastrovascular cavity, partitioned with internal sheets or septae. There is no medusoid phase for this class, so alternation of generations does not occur.
Examine the demonstration specimen of Metridium, a large solitary sea anemone. Locate the tentacular crown, body stalk and pedal disk. Follow the mouth down the pharynx to the gastrovascular cavity. Notice that the cavity is divided by a series of septa. What function do these septa perform? Is there an anus? Is this a colony or one individual?
Observe the demonstrations of representative hard corals. These hard or stony corals have cup-like depressions called calyxes that house the polyps. Notice the septae that correspond to the septae found in the gastrovascular cavity of the polyp. They provide additional surface area affording the polyp a more secure seat. The coral skeleton itself is made of calcium carbonate (CaCO3) that is secreted by the undersurface of the polyps.
Observe the demonstration of soft corals: sea fans and sea whips. Under a magnifier or dissecting microscope locate the tiny calyxes similar to those of the hard corals. Soft corals have two skeletons: a central horny or wood–like axis and a matrix of spicules surrounding this axis.
PHYLUM CTENOPHORA (COMB JELLIES)
The comb jellies used to be classified with the cnidarians as the Phylum Coelenterata, meaning "bag animals". Like the cnidarians they are diploblastic, with a bag-like, radially symmetrical architecture and a simple, distributed, web-like nervous system. The distinguishing features of the ctenophorans are 1) eight comb-like rows of cilia which beat rhythmically to propel the animal and 2) a pair of tentacles armed with adhesive structures called colloblasts.
Observe the preserved comb jelly on display and the video of live comb jellies. Look for the combs of beating cilia. Is the ciliary beating pattern synchronized or organized in any manner that you can identify? How does this animal propel itself?
GROUP IIB BILATERIA:ACOELOMATES
These phyla are bilaterally symmetrical and triploblastic. The body interior is filled with solid, spongy mesenchyme tissue and lacks a body coelom or internal body cavity. Flatworms are the only representatives of this group which we will study.
PHYLUM PLATYHELMINTHES (FLATWORMS)
The Platyhelminthes are quite literally the “flat” (=platy) “worms” (=helminth). These are dorsoventrally compressed, triploblastic acoelomates. Each class is quite unique. The free-living members of Class Turbellaria are most representative of the ancestral condition; the other three classes show a wide variety of specializations that go along with the evolutionary move to a parasitic way of life. Most, but not all, trematodes are hermaphroditic (monoecious) - containing both male and female sexual organs and reproductive tracts.
Class Turbellaria (free-living flatworms)
Examine the prepared slides of Planaria stained to show the digestive system. Locate the mouth, pharynx (proboscis) and the branched intestine. What advantage does this system show as compared to a simple sack intestine? Find the light-sensitive “eyes”.
Examine a slide with a cross-section of a planarian. Find the intestine and the mesenchyme, a loose aggregation of cells that fills the space between the outer body wall and internal organs. Note the total absence of a body cavity.
Observe the live planaria on display. Conduct a simple experiment to determine how they react to light. Are they positively or negatively phototaxic? How does this relate to their role as scavengers and detritus-feeders?
Classes Trematoda (digenetic flukes) and Monogenea (monogenetic flukes)
Flukes are parasites, some of which have complex life cycles involving two or more hosts. Monogenic flukes are principally ectoparasites with vertebrate hosts. They cling to well vascularized animal surfaces such as fish gills, mouth cavities, and the peripheral urogenital system, using an enlarged opisthaptor . The opisthaptor may be armed with multiple suckers and/or hooks. As the name implies monogenetic trematodes have a simple life cycle progressing from a parasitic adult to eggs to free-swimming larvae and back to parasitic adults.
Digenetic flukes are endoparasites whose life cycles involve more than one host. Adult digenetic flukes typically inhabit vertebrate "definitive" hosts and live in locations ranging from the circulatory system to the bile ducts of the liver. Eggs are passed to the outside and develop into free-swimming larvae. These infest an invertebrate "intermediate host”, generally a fresh-water snail or marine annelid. In the invertebrate host they form cysts in muscle, brain, heart, lungs, gonads, or other highly vascular tissue. These cysts may passed to the vertebrate host when it ingests the infected invertebrate or they may hatch into free-swimming larvae which burrow or are ingested into the host. Many flukes are parasitic on humans and domesticated animals, hence they are of medical importance to ourselves and many of the animals we raise for food production.
Examine the whole mount slides of the human liver fluke Opisthorchis sinensis (= Clonorchis) and the larger sheep liver fluke Fasciola hepatica. Locate both the both the oral and ventral suckers. Trace the digestive system from the anterior sucker surrounding the mouth, to the pharynx and branched intestine. Trace both the male and female reproductive tracts.
Observe the slides of the dioecious, sexually dimorphic, human blood fluke Schistosoma mansoni. The female is the smaller animal clasped in copulo in the long ventral folds of the male.
Class Cestoidea (tapeworms)
The cestodes are a group of intestinal endoparasites of both vertebrate and invertebrate hosts. As with the flukes, they are important in relation to human and farm animal health. Tapeworms are divided into distinct segments called proglottids. Each proglottid contains a complete set of both male and female reproductive organs. New proglottids form in the neck region, while the oldest and most mature gravid (egg-producing) proglottids are found at the posterior end of the body.
Examine the prepared slides of the tapeworm Taenia pisiformis, an intestinal parasite of mammals including humans. Find the head or scolex. What types of attachment devices are found on the scolex? Find the individual segments or proglottids. Locate an immature proglottid near the anterior end, one in which none of the internal organs have clearly formed. Now find a mature proglottid near the middle of the animal. The structures you see are almost all for the purpose of sexual reproduction. Finally, find a gravid proglottid near the posterior end of the animal. Locate the egg bearing capsules that fill the swollen uterus. Notice that there is no digestive tract in this animal. How does it nourish itself?
Tapeworms are also digenetic, although both the intermediate host and the final host may be vertebrates. As discussed in class, the larval forms of some tapeworms form fluid-filled hydatid cysts in the internal organs of the intermediate host. These cysts can be quite large and debilitating. The larval form is generally passed to the final host when the weakened intermediate host is killed and eaten.
GROUP IIC BILATERIA:PSEUDOCOELOMATES
These phyla are bilaterally symmetrical and triploblastic. They have an internal coelom, but it is not lined with mesoderm.
PHYLUM ROTIFERA (WHEEL ANIMALS)
Rotifers are microscopically tiny, multicellular organisms. They get their name from the wheel-shaped, ciliated jaws which sweep particulate food into the mouth. In spite of their small size they are anatomically fairly complex. Their digestive system is double-ended and they have an unlined internal body cavity. Many reproduce by parthenogenesis; that is all individuals are females which develop from and produce diploid, unfertilized eggs.
Observe the rotifer slide and live rotifers on display. Look especially for the rapidly-beating cilia of the circumoral wheel organs.
PHYLUM NEMATODA (UNSEGMENTED ROUND WORMS)
The nematodes comprise a large number of taxa that include free-living forms as well as both plant and animal parasites. This phylum is a very successful group and their existence contributes to the welfare of humans in both positive and negative ways.
Observe the preserverd male and female Ascaris specimens. These worms are intestinal parasites of mammals including humans. This species demonstrates sexual dimorphism with the males having a hooked posterior and bearing sharp spines called copulatory spicules near the posterior tip.
Observe the live sample of vinegar eels under a dissecting microscope. These animals have longitudinal muscles running the length of the animal, but no radially-arranged muscles around the circumference. Observe their wriggling. Which of the following motions are present: lateral flexing, coiling, undulating waves of contraction which run from one end of the animal to the other? It may be necessary to put a few of these animals in a depression slide and slow them down with some Proto-Slo.
GROUP IID BILATERIA; SCHIZOCOELOMATES
These phyla are bilaterally symmetrical and triploblastic. They have an internal coelom lined with mesoderm which forms by splitting solid blocks of mesoderm.
PHYLUM MOLLUSCA (MOLLUSKS)
The mollusks are the second largest phylum of animals behind the Arthropoda. Included in the Mollusca are many organisms found commonly along the sea coast, in the garden, and in fresh water streams and ponds. The group is highly diverse and successful with numerous organisms that display unusual, often bizarre, adaptations that have allowed them to compete successfully in an evolutionary sense. Most are dioecious, having separate male and female individuals.
Mollusks are characterized by an unsegmented body mass which includes a muscular foot, a visceral mass containing the organs which are often twisted in a pattern called torsion, and a specialized fold called the mantle. The mantle houses a mantle cavity containing the gills and is connected to the outside world via tube-like incurrent and excurrent siphons. In some groups the mantle secretes a calcareous shell. The mouth has a distinctive file-like structure called the radula.
We will study four major classes of molluscs in this lab.
Class Polyplacophora (chitons)
Chitons are marine animals with a relatively simple body plan, probably similar to that of the common molluscan ancestor. The body is covered with a shell compose of eight plates. Note that the body itself is NOT segmented.
Examine both the preserved chiton specimens on display. The chiton's foot serves as an effective suction cup to hold it to the surface upon which it grazes.
Class Gastropoda (snails and slugs)
The gastropods (literally "stomach-foots") inhabit marine, freshwater, and terrestrial habitats. Both shelled (snails, whelks, etc.) and unshelled (slugs and nudibranchs) forms exist. They are characterized by a distinct head containing cerebral neural ganglia and a pair of eyes on extensible stalks.
The shelled forms of gastropods dramatically exhibit the secondary developmental pattern called torsion. This process rotates the anus up over the head and results in a spiraled, somewhat asymmetrical animal. Examine the diversity of form in shells of the marine gastropods on display here. Both planospiral and helically coiled types are present.
Examine the marine snails in the lobby aquarium. The path of a snail can often be traced by the trail of slime which it leaves behind. In terrestrial snails this slime also forms an effective plug which seals the animal in its shell during dry periods and prevents desiccation.
Examine the display of shell color variants in a single species of terrestrial snail. Land snails often show extreme phenotypic color polymorphisms.
Class Bivalvia or Pelecypoda (clams, mussels, and oysters)
Bivalves have two shells which are connected by a dorsal hinge. Most are relatively sessile filter-feeders, extracting plankton from water which is pumped through the gills and out of the siphon. Some species are ectoparasites of fish, especially of the fins and gills. When disturbed many clams can propel themselves for short distances backwards by repeatedly snapping the shells together and/or forcefully expelling water through the excurrent siphon.
Examine clam shells on display and relate these to the clam anatomy model. In the model locate the two adductor muscles, which keep the clam closed. Locate the sheet-like mantle fold, the secretory organ that produces all three shell layers. It can be found adhering to the inner surface of the shell. Find the lamellar gills and the muscular foot. What functions do each of these structures perform? Find the major components of the shell including the umbo (center of growth), the hinge line, the elastic ligament which serves to open the shells, and the adductor muscle scars which are cuplike depressions in the inside of the shell that mark the positions of the adductor muscles.
Class Cephalopoda (cuttlefish, nautilus, squids, and octopi)
The cephalopods, as the name suggests, have the most elaborate, centralized, and cephalized nervous systems of all of the invertebrates. They also exhibit a radial organization of the arms and the associated peripheral nervous system.
Observe the demonstration specimens of the squid Loligo . Locate the muscular sac-like mantle. Find the head with the circumoral ring of arms and tentacles and well developed paired eyes. The arms are relatively short and have suckers along their entire length. The longer single pair of tentacles have suckers located only at the expanded distal portion called the club. Find the excurrent siphon on the ventral surface of the body at the junction between the head and mantle. The funnel is used to direct the flow of expelled water used for “jet propulsion.”
Observe the demonstration of another cephalopod, Octopus. How does this animal differ from Loligo? List several features.
Observe the chambered nautilus shell. How does this shell differ from that of a typical gastropod?
Finally, observe the "cuttlebone" from a cuttlefish. Is this an external or an internal shell?
PHYLUM ANNELIDA (SEGMENTED ROUNDWORMS)
The Phylum Annelida includes the segmented worms. In addition to metamerism, the serial repetition of body parts, the annelids display a closed circulatory system, a spacious coelom (schizocoel) that acts as a hydrostatic skeleton, a high degree of cephalization, and a well developed, muscular body wall. The group is triploblastic. The annelids were formerly divided into three classes; Polychaeta, Oligochaeta, and Hirudinea. However, the current molecular systematics adopted by your text is to group them as two classes - the free-swimming Errantia, and the more sessile Sedentaria.
Class Errantia:Polychaeta (true polychaetes)
The polychaetes bear paired lateral appendages called parapodia. They have a well–developed head that typically bears a variety of sensory structures. The polychaetes are dioecious but lack discrete gonads. The gametes arise directly from the peritoneum, the lining of the coelom. Development is indirect, i. e. a larva is produced. There is no clitellum and copulation does not take place. Instead broadcast spawning is followed by external fertilization. The group is marine.
Observe the specimens of Nereis, the clamworm or sandworm. Locate the paired, segmentally arranged appendages, the parapodia. Examine the well developed head region composed of two modified segments. There are a variety of sensory structures located on the head including pigmented eyes and elongated tentacles. Locate these structures. Also locate the large, paired jaws at the rostral (head) end.
Class Sedentaria:Canalipalpata (fanworms)
The "fanworms" or "bristle-footed worms" are sessile, tube-dwelling marine annelids. The parapodia are modified into rings of ciliated palpi which are used in filter-feeding. The fanworms were formerly formerly grouped with the free-swimming marine polychaetes, but recent molecular systematics suggests that they are more closely related to the earthworms and leeches.
Examine the fanworm in the lobby aquarium. Note that the fan is composed of modified segmental parapodia and is used for filter-feeding microscoppic plankton.
Class Sedentaria:Oligochaeta (earthworms)
In the next lab, you will study features of the digestive, circulatory, excretory and reproductive systems of the earthworm Lumbricus terrestris.
For now examine closely the live specimens on display. What are some of the major differences between this animal and Nereis? Which do you feel is more typical of the phylum as a whole. Why?
Class Sedentaria:Hirudinea (leeches)
The class Hirudinea includes the leeches. Some leeches are predators which feed on small microcrustaceans and some are ectoparasites that feed on the blood or lymph of a host organism. The pharynx is modified as a pumping organ to suck in these juices. The mouth is surrounded by an adhesive sucker and a second sucker may be located near the posterior end of the body. Why? One anatomical difference between the leeches and the other annelids is that leeches lack complete septae internally separating adjacent body segments.
Examine the external morphology of the leech placed out on demonstration. Note that it lacks both appendages and setae. Find the anterior and posterior suckers. What is the function of each?
PHYLUM ARTHROPODA (JOINTED-LEGGED ANIMALS)
The Phylum Arthropoda is the largest and most successful group of animals on earth. In fact, this phylum includes about 3/4 of all species that inhabit this planet. They have successfully conquered both aquatic and terrestrial areas and have mastered flight as well.
The group includes the horseshoe crabs, spiders, shrimp, lobsters, barnacles and insects.
The basic features of the phylum include a chitonous exoskeleton with paired, jointed appendages; a segmented body often consolidated into three main regions called tagmata (head, abdomen, and thorax); a reduced coelom; complete digestive system; open circulatory system; and a high degree of cephalization with a broad array of complex sense organs. Arthropods undergo a process called molting as they grow. Many arthropods also pass through one or more distinct, structurally different stages, a process called metamorphosis.
Many arthropods have very complex patterns of behavior. Some have developed a social community structure and polymorphism. Due to the size and diversity of this group, extreme derivations and modifications of the basic body plan are common.
Subphylum Chelicerata
The chelicerates include a diverse assemblage of organisms including spiders, mites, ticks, scorpions, and the horseshoe crab, which in reality is not a crab at all. All these organisms lack antennae and mandibles. The first segmental pair of appendages called chelicera are teh primary mouthparts. The body is generally divided into two distinct multi-segmental tagmata: and anterior cephalothorax and a posterior abdomen.
Examine the demonstration specimens and live specimens of the horseshoe crab, Limulus polyphemus, a member of the Class Merostomata and a so-called "living fossil" from the Ordovician Period. Locate the carapace with its dorsal compound eyes. Flip the specimen over and count the seven pairs of jointed legs. Locate the book gills and telson (caudal spine) on the abdomen. How do the book gills get their name?
Examine the preserved and live specimens of a typical member of the Class Arachnida, the American tarantula. Count the six pairs of legs; from front-to-back the chelicera (modified to form venomous fangs), the pedipalps, and four pairs of walking legs. This is probably best done on the preserved specimen! Note the multiple compound eyes on the cephalothorax and the spinnerets at the tip of the abdomen. At some point in the class your instructor will feed a cricket to the tarantula so that you can watch this predator in action.
Examine the preserved specimen of the scorpion, another terrestrial arachnid. Notice that the second pair of legs, the pedipalps are modified into large claws. The caudal abdominal segments are modified into a tail tipped with a venomous stinger.
Subphylum Myriapoda
The myriapods have modified mandibles (the second segmental pair of appendages) as the primary mouthparts and multiple body segments, not fused into tagmata. The two main Classes are Chilopoda and Diplopoda.
Carefully examine the mounted centipede (Class Chilopoda) and the live millipede (Class Diplopoda). What are some of the major structural and ecological differences between these two classes?
Subphylum Pancrustacea
The Subphylum Pancrustacea includes the predominantly marine and fresh-water crustaceans and the predominantly terrestrial insects.
Subph Pancrustacea:SuperClass Crustacea
The crustaceans are a large and successful group of organisms that include many common animals; indeed many that we consider good to eat such as the shrimp, crabs, lobsters, etc. In addition this group also includes the barnacles (once considered to be in the Phylum Mollusca), pill bugs, and water fleas. Crustaceans have two pairs of antennae and multiple pairs of biramous appendages (= "two-branched") on both the cephalothorax and the abdomen. At least three pairs of these most rostral legs are modified feeding appendages. Both crustaceans as well as uniramians (the next group) have mandibles rather than chelicerae.
Crustaceans range in size from microscopic to several kilograms. Major groups include the decapods (crayfish, lobsters, shrimp, and crabs), branchiopods (fairy shrimp and daphnia), copepods, ostracods, isopods (pill bugs), and amphipods (water fleas).
Obtain a live specimen of the crayfish Procambarus clarkii (Class Decapoda) for examination. Observe that the body is divided into two regions, the cephalothorax and the abdomen. Locate the long antennae, shorter antennules, and the stalked eyes attached to the head. Most crustaceans have a shell-like chitinous carapace covering most of the cephalothorax. The point of the carapace extending anteriorly between the eyes is called the rostrum. Note that the mouth is surrounded by a series of specially modified, serially arranged mouth parts. The pair closest to the mouth are the mandibles. Locate the large pincers or chelae on the anterior set of walking legs. Locate the abdominal legs or swimmerets. What is their function? Locate the fan-like tail of the body. The anus is located ventrally at the base of the central telson. Tap on the tail of the crayfish. It may exhibit a tail-flip, which is an extremely fast, stereotyped, escape maneuver.
Observe the preserved specimens of other decapods, including crabs, shrimp, and lobsters.
In Lab 10 you will have a chance to observe several classes of "microcrustaceans harvested from Foster Lake. These will likely include Cl. Branchiopoda (Daphnia), Cl. Cl. Copepoda (Cyclops), and Cl. Ostracoda. For now, exam these on the arthropod diversity poster.
One rather deviant group is the Class Cirripeda, a.k.a the barnacles. The small fan-like appendages protruding from the conical shell are the legs, which function to sweep in particulate food. If a barnacle doesn't look like a crustacean to you, think of it this way: a barnacle is nothing more than a small crustacean sitting on her head, with her carapace pulled in around him, and waving her feet out in the water.
Subph Pancrustacea:Class Insecta
Adult insects are characterized by a segmented body divided into three tagmata or parts; head, thorax, and abdomen. The head contains compound eyes, antennae, and a mouth with well-developed mandibles. The thorax contains six pairs of legs and two pairs of wings (in most insect orders). Respiration is through tube-like tracheae which open onto the abdominal surface through spiracles.
Examine the demonstrations of several orders within the Class Insecta. In Lab #10 you will be collecting immature forms of many of these from Wolf Creek in the Arboretum. Which insects exhibit complete metamorphosis (indirect development - see below) and which exhibit incomplete metamorphosis (direct divelopment - see below)? Be able to trace the steps in both complete and incomplete metamorphosis, including the juvenile forms in each.
Pay particular attention to the beetles of Order Coleoptera. This is an extremely popular body design; there are more known species of beetle that of all other animals combined.
Examine the live Madagascar "hissing cockroach." Poke it or pick it up to discover how it gets its name. The startling sound you hear is produced by air being forced out through the spiracles.
GROUP IIE BILATERALIA:ENTEROCOELOMATES
This final clade of phyla are also bilaterally symmetrical and triploblastic. They have an internal coelom lined with mesoderm which forms from an outpocketing of the embryonic gut, the archenteron. Unlike the other eumetazoan phyla which you have studied, the enterocoelomates are deuterostomic, meaning that the blastopore eventually becomes the anus of the adult animal. There are three principal deuterostomic Phyla: Echinodermatata (the "spiny-skinned" animals), Hemichordata (the acorn worms and pterobranchs), and Chordata (the lancelets, tunicates, and vertebrates). You will study the diversity of this group, to which the vertebrates formally belong, in lab #6.
PART II. INVERTEBRATE LIFE CYCLES
Materials
Diagrams of life cycles for representative invertebrates
Live, mounted (slide), or photographic specimens of life history stages
Procedure
1. Work through the life cycle of each of the invertebrate specimens and practice ordering the reproductive processes and life stages in the biological sequence.
2. For each reproductive process, determine what environmental conditions trigger the process; i.e. to what environmental conditions is this reproductive process and evolutionary adaptation?
3. For each life history stage be able to answer basic questions such as:
a. How is does this stage obtain nourishment and grow?
b. Does this stage require specific interactions with a host organism?
c. How does this stage "produce" the next stage?
4. Work with any live organisms that are provided and relate these to the species life history.
Study Suggestions
1. Make detailed sketches and notes on the life cycles. This will help you to look at the specimens more closely, as well as to help you study later.
2. Practice scrambling and reorsering the life history photos, then check yourself against the life cycle diagrams.
Phylum Cnidaria: Obelia
Obelia is a marine member of Class Medusazoa:Hydrozoa. Like many Cnidarians, its life cycle alternates between two diploid stages, a sessile polyp colony form and a free-swimming medusa form. As you saw earlier in the lab, the polyp form develops as a highly branched tree-like "colony" with both vegetative (feeding) polyps (gastrozooids) and reproductive polyps (gonozooids). Asexual reproduction takes place by budding off of a vegetative polyp, which settles to the substrate and forms a new colony. The reproductive polyps produce tiny male and female medusae which are released into the water and reproduce sexually.
Reexamine the prepared slide of Obelia and compare it to the life cycle diagram. Be sure that you can distinguish vegetative and reproductive polyps and the roles that each plays. Understand that, unlike the fungi (and the plants), both stages or generations in the life cycle are diploid.
What local conditions do you think would favor asexual vs. sexual reproduction? Hint: if food is locally abundant would producing an identical genetic copy of yourself (polyp asexual bud), or sharing your genes (free-swimming sexual medusa) be a more efficient way to maximally contribute your genes to the next generation?
Phylum Platyhelminthes: Fasciola hepatica
Fasciola hepatica is a digenetic trematode (Class Trematoda), whose parasitic life cycle involves two hosts, and several stages of both free-swimming and endoparasitic larval forms. The adult liver fluke (1) lives in the bile ducts of the vertebrate definitive (primary, final) host; a human, sheep, or cattle. Eggs (2) are passed with the bile into the host's hindgut and out with feces. The eggs hatch into free-swimming ciliated miracidia (3) larvae which infest the intermediate (secondary) host - a fresh-water snail. Each miracidium forms a sporocyst, which then produces several rediae (4) larvae. Each redia produces several free-swimming cercaria (5) larvae which leave the snail, attach to aquatic vegetation, and form walled metacercariae (6) cysts. When a metaceraria cyst is inadvertently eaten by a sheep, cow, or human, it excists in the duodenum of the small intestine, travels up into the bile ducts, and establishes itself as an adult liver fluke, completing the cycle. The life cycle of Fasciola may be understood as an extremely clever way for an adult fluke trapped deep inside the liver of one mammal host to get the maximum number of its genetic offspring into the livers of the maximum number of other mammal hosts.
A set of six microscopes are set up, surrounding a diagram of the Fasciola life cycle. Compare each microscopic specimen to the photograph card. The eggs and larval forms are very tiny and may be difficult to find on the slide. What are the unique adaptive features of each reproductive form?
1) Adult fluke. Note the oral feeding sucker and the ventral attachment sucker. The blind gut is simple and bifurcated (as you saw in the Planaria) and difficult to see. The bulk of the rest of the interior of the fluke is taken up by the anterior uterus, the lateral shell glands, and the posterior paired testes. The adult fluke is really designed to take advantage of the nutrient-rich enviromnment of the bile duct and to function as a maximally efficient egg-making machine. See how many of the reproductive structures labeled on the accompanying diagram you can identify in the actual fluke.
2) Eggs. Find the small, pale yellow eggs on the eggs slide.
3) Miracidia. Try to make out the cilia on the tiny, purple free-swimming miracidia larvae.
4) Rediae. The rediae larvae have an elongated worm-like appeance. These are a parasitic stage within the tissue of the snail secondary (intermediate) host. Within each greenish redia look for the tiny red cercaria larvae developing.
5) Cercaria. The cercaria are another free-swimming form which burrows out of the infected snail. Note the motile tail, which gives the cercaria its tadpole-like appearance.
6) Metacercaria. When the cercaria contacts a suitable plant leaf it forms a walled cyst called a metacercaria and awaits ingestion by its next mammalian final host. Note the spherical cyst wall and the tiny flukelet lurking inside. Digestive enzymes in the mammalian host rupture the cyst wall and allow the new fluke to "excyst" and head for the liver.
Trace the fluke life cycle a few times, until you are comfortable with the sequence of larval forms. Human practices which promote infestation with this fluke include 1) using human and animal waste for fertilizer, 2) using cattle to pull plows, 3) growing crops (such as rice) in flooded fields, and 4) not washing crops before eating them. How does each of these practices contribute to maintaining the fluke life cycle and involving humans as the final host?
Phylum Arthropoda: Nasonia vitripennis
Nasonia vitripennis is a tiny parasitoid wasp of the insect Order Hymenoptera. A parasitoid is an animal which, during its own development, slowly consumes its host - basically the premise of the sci-fi classic movie "Alien". Nasonia females oviposit (lay eggs) within the puparium (pupal case) of a much larger fly host. The eggs rapidly hatch into maggot-like larvae, which slowly devour the paralyzed host fly pupa. After all of the edible parts of the host have been consumed, the wasp larvae metamorphose into wasp pupae, still within the puparium (pupal case) of the host fly. The pupae subsequently metamorphose into adult wasps which tear out of the fly puparium and mate almost immediately. The mated female wasps fly off to find another fly puparium and the flightless males are left behind to die.
Study the life cycle diagram for the wasp Nasonia and its fly (Sarcophaga) host, along with the accompanying photographs.
How do the life cycles of fly and wasp intersect? Are both of these insects exhibiting complete or incomplete metamorphosis?
Use your thumbnails to carefully open a fly puparium containing Nasonia parasites. Examine the parasites under a dissecting microscope. Can you determine the developmental stage of the parasites - larva, early pupa, late pupa?
There are four closely-related species of Nasonia, and each has its own unique courtship ritual. Male wasps can mate several times, but females mate only once, so courtship will only be seen in virgin females. If adult virgin wasps are available the instructor will help you set up a chamber for observing wasp courtship and mating. Add one or more male and one or more virgin female wasps to the chamber, close it, and immediately begin observing. The male and female should court and mate as soon as they contact each other. Can you determine any of the signals, steps, or "moves" in this brief, intense courtship "dance"? Do courtship rituals constitute a prezygotic or a postzygotic reproductive isolating mechanism?
Watch the video of mating in the parasitoid wasp Melittobia digitata. The male is pale, wingless, eyeless creature circulating from female to female. What are the steps in this courtship? What signals do the male and female seem to be using? How does a female signal that she is not sexually receptive?
If mated female wasps are available the instructor will help you set up a chamber for observing ovipositing behavior. Add one or more fly puparia and one or more female wasps to the chamber, close it, and begin observing. How does the wasp inspect the fly puparium? What do you think that she is looking/feeling/smelling for? Can you observe her drilling a hole through which to oviposit? If adult females are not available the instructor will run a video of the puparium inspection and oviposition process.
One additional interesting feature of Nasonia biology is that they are haplodiploid, like all other members of Order Hymenoptera. Females develop from fertilized eggs and are diploid, while males develop from unfertilized eggs and are haploid. So, females have both a genetic mother and father, while males have only a mother. How do you think that this might affect the reproductive "strategies" of males and females in their life cycles?