BIO112 Laboratory Guide #6
DIVERSITY OF THE INVERTEBRATES II
INTRODUCTION
In the previous lab you looked at representative major invertebrate phyla from the Parazoa (Ph. Porifera - sponges), Radiata (Ph. Cnidaria - hydras, jellies, anemones, corals; Ph. Ctenophora - comb jellies), Acoelomates (Ph. Platyhelminthes - flatworms), Pseudocoelomates (Ph. Nematodes - roundworms; Ph. Rotifera (wheel animals); and Schizocoelomates (Ph. Mollusca - mollusks; Ph. Annelida - segmented worms; Ph. Arthropoda - jopinted-legged animals). It might be useful at this time to review the defining developmental and structural features of each phylum, as well as the subphyla and classes and the representative members that you saw in the lab. You also looked in some detail at the life cycles of three invertebrates - a hydrozoan, a digenetic trematode, and two ecologically intertwined insects, the flesh fly and the parasitoid jewel wasp.
In the first part of his week's lab you will complete our survey of the invertebrates by examining the other main group of true coelomates, the Deuterostomes. You should recall from our class work on animal phylogeny that the deuterostomes developmentally exhibit:1) radial and indeterminate cleavage
2) development of the anus from the blastopore (hence "deuterostomic" development of the mouth), and
3) formation of the coelom by enterocoelous budding of mesodermal sacs from the primitive gut.
We will look at four main deuterostome clades: Ph. Echinodermata (spiny-skinned animals), Ph. Hemichordata (acorn worms and pterobranchs), Ph. Chordata : SubPh. Cephalochordata (sea lancelets), and Ph. Chordata : SubPh. Urochordata (sea squirts). Thes latter three groups are often collectively called the "protochordates" because of their structural and phylogenetic affinity with the remaining subphylum of Chordata, Subphylum Vertebrata (the vertebrates). The vertebrates will be the subject of the next two labs.
In the second part of this lab you will perform virtual dissections (close microscopic examination) or actual dissections on a representative member of most of the major animal taxa which you have studied.
After completing this laboratory you should be able to:
1) outline and describe in appropriate terms the diversity of the Deuterostome clade of animals.
2) describe the basic body plans of the major deuterostomic phyla.
3) group these phyla based on central features of the body plan and developmental patterns using both traditional and modern 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 organ systems and major organs in representative members of the invertebrate taxa.
PART I. DIVERSITY OF THE DEUTEROSTOMES
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 deuterostomic invertebrate phylum and many major classes are on display. These specimens include both preserved animals 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. The lobby aquarium may have live representatives of some of these groups, specifically the echinoderms. If specimens have been isolated for you to observe, handle them with care and do not keep them out of their salt-water environment for more than 1 minute at a time.
3. Be able to describe the distinguishing features of each taxa of deuterostomic invertebrates. Within the echinoderms be able to recognize the five classes.
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 pentaradial body
2. asymmetrical body; sessile, filter-feeding adult
3. lacking true tissues or nervous system Ph. PORIFERA [lab #5]
3. coelomate; pharynx with gill slits Ph. CHORDATA;SubPh. Urochordata
2. radially or pentaradially symmetric body
3. radial symmetry, bag-like body; diploblastic
4. stinging cells on tentacles Ph. CNIDARIA [lab #5]
4. ciliated combs, adhesive cells on tentaclesPh. CTENOPHORA [lab #5]
3. pentaradial body; triploblastic; enterocoelomatePh. ECHINODERMATA
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 [lab #5]
3. pseudocoelomate or coelomate; two-ended digestive tract
4. pseudocoelomate
5. body elongated and worm-like Ph. NEMATODA [lab #5]
5. microscopic; mouth bearing ciliated wheel-organsPh. ROTIFERA [lab #5]
4. coelomate
5. dorso-ventrally paired shells; enterocoelomatePh. BRACHIOPODA [lab #5]
5. shell single, laterally paired, internal or absent; schizocoelomate;may show visceral torsion or radial limbs
Ph. MOLLUSCA [lab #5]
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 [lab #5]
3. schizocoelomate or enterocoelomate
4. schizocoelomate
5. open circulation
6. jointed appendages Ph. ARTHROPODA [lab #5]
6. non-jointed appendages Ph. ONYCHOPHORA [lab #5]
5. closed circulation Ph. ANNELIDA [lab #5]
4. enterocoelomate
5. dorsal notochord Ph CHORDATA; Subph. Cephalochordata
5. no notochord; prominent everted proboscisPh. HEMICHORDATA
GROUP IIE BILATERIA; ENTEROCOELOMATES
Like the protostomic schizocoelous phyla (Mollusca, Annelida, Arthropoda) which you studied last week, the deuterostomic enterocoelous phyla are bilaterally symmetrical and triploblastic. They have an internal coelom lined with mesoderm which forms from outpocketings of the embryonic gut (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.
PHYLUM ECHINODERMATA (SPINY-SKINNED ANIMALS)
Echinodermata is the most speciose and diverse extant deuterostome phylum of invertebrates. The entirely marine group includes the familiar starfish, sea urchins, sand dollars, brittlestars, sea cucumbers, and sea lilies. The echinoderms have bilaterally symmetrical larval forms, however the adults are "seconarily pentaradially" symmetrical animals with calcareous endoskeletons. The individual skeletal elements called ossicles may be variously modified or fused into a central sack-like test. A unique feature of the echinoderms is the water vascular system which plays a vital role in locomotion, respiration, food acquisition and sensory perception.
Live specimens may or may not be available for some of the echinoderm classes.
Class Asteroidea (sea stars)
The asteroids are the common starfishes. They are characterized by arms which are not distinctly separated from the central disk and which have an open ambulacral groove on the under surface.
Observe the demonstration specimen of Asterias. Externally, locate the central disk and the arms. Find the oral (mouth bearing) and aboral (non–mouth bearing) surfaces. Find the madreporite and anus on the aboral surface. What is the function of each of these? On the oral surface locate the mouth, the open ambulacral grooves that run down each arm, and tube feet that extend out of the ambulacral grooves.
Examine a living sea star closely under a dissecting microscope. Find the light-sensitive oceli at the end of each arm. If you look closely at the dorsal skin you may be able to see the tiny pincer-like pedicellaria which keep the surface of starfish free of debris and ectoparasites. Return the sea star to a large finger bowl full of seawater, but place it upside down. How does it right itself? (This process may take several minutes). How do the tube-feet function in this process?
Class Ophiuroidea (brittlestars, serpent stars, and basket stars)
The ophiuroids are easily distinguished from the asteroids (true starfishes) by the clear separation of the arms from the central disk and the absence of ambulacral grooves.
Observe the preserved and live brittle stars. How does its structure and mode of movement differ from the sea stars?
Class Echinoidea (sea urchins and sand dollars)
The echinoids include the sea urchins and sand dollars. The ossicles are fused to form a rigid, shell-like, central test.
Observe preserved and live sea urchins. Notice that the entire test is covered with spines. Locate the mouth in the center of the oral surface. Around the mouth is located a flexible sheet of tissue, the peristomial membrane. How do the tube-feet and spines cooperate in moving the animal along the substrate?
Examine the hard tests of sand dollars and sea urchins. The five-pointed (pentaradial) architecture is readily apparent in these. Find the complex jaw apparatus called Aristotle’s lantern in the plastic sea urchin mount.
Class Crinoidea (sea lilies)
At first glance sea lilies may also not look like the other echinoderms. Unlike the other echinoderms the oral surface is oriented upwards. Five feathery arms, each with several branches, extend upwards from a central calyx or body. Multiple tube feet used in locomotion and in suspension-feeding radiate out from pinnules, which in turn extend out from the arm branches.
Examine the preserved sea lily specimens to familiarize yourself with the basic body orientation and confirm the pentaradial architecture. Examine the fossilized test and "stem" segments.
Class Holothuroidea (sea cucumbers)
The sea cucumbers are unusual organisms and somewhat atypical echinoderms. The body is expanded along the oral-aboral axis and the organism lies on its side on the bottom the sea.
Examine the preserved and live specimens of sea cucumbers. The oral end has a crown of short tentacles surrounding the mouth. The pentaradial architecture is expressed by the five rows of short tube feet running the length of the animal.
PHYLUM HEMICHORDATA (ACORN WORMS AND PTEROBRANCHS)
The hemichordates exhibit pharyngeal gill slits, a trait they share with the much more specious and diverse chordates. They are morphologically divided into an anterior proboscis, a middle collar surrounding the mouth, and a posterior body. Most hemichordates are tube-dwelling filter-feeders.
Examine the preserved and mounted acorn worm specimens to identify these structures.
PHYLUM CHORDATA (ANIMALS WITH A NOTOCHORD)
The chordates are characterized by the presence of four chordate structures, at least in the larval forms: pharyngeal gill slits, a notochord, a dorsal hollow nerve chord, and a post-anal tail. This group includes both invertebrates and vertebrates. Unlike the vertebrates, the invertebrate chordates exhibit little if any cephalization (centralization of nervous tissue in the head). The two invertebrate chordate subphyla are structured rather differently, but both are filter-feeders.
Subphylum Cephalochordata (sea lancelets)
Free-swimming adult sea lancelets exhibit all four of the defining chordate characteristics.
Examine the preserved specimen of the lancelet Branchiostoma (formerly Amphioxus) under a dissecting microscope. Identify the notochord, pharyngeal gill slits, and mouth, surrounded by buccal cirri. Notice also that the muscle in this animal is clearly divided into blocks, revealing the segmented body plan characteristic of both invertebrate and vertebrate chordates.
Lancelets exhibit much more efficient swimming than what you saw in either the vinegar eels (nematode) or leech (annelid). What structure allows this efficient transfer of force from the muscles to the water (hint: it's the "chord" in "chordate")?
Subphylum Urochordata (tunicates or sea-squirts)
Free-swimming tunicate larvae ("ascidian tadpoles") also exhibit all four of the chordate characteristics. However, adult tunicates are sessile bag-like animals which retain only the pharyngeal gill-slits.
Examine preserved specimens of both larval and adult tunicates. In the larva try to identify the notochord, nerve chord, pharyngeal gill slits, and tail. In the adult identify the pharyngeal basket, incurrent mouth, and excurrent atrial siphon.
Subphylum Vertebrata (vertebrates)
The vertebrates are currently by far the most successful and diverse subphylum of Chordata, and indeed of the deuterostomes. We will be studying the vertebrates in some detail in the next two labs.
For now, examine the "ammocetes" larva of the lamprey, a comparatively "primitive" vertebrate. Compare it to the larval tunicate and the adult lancelet and note its fundamental similarity of design. Can you find all four of the chordate characteristic structures?
PART II. COMPARATIVE INVERTEBRATE ANATOMY
Materials
Prepared slides, mounted specimens, models, and preserved specimens ofrepresentative invertebrates
Guides to anatomy and dissection of each organismCompound and dissecting microscopes; magnifiers
Dissecting trays and instruments
Procedure
For most of the major metazoan invertebrate phyla a single representative species is available for closer anatomical examination and/or dissection.
1. For mounted (slide or plastomount) or model specimens, work through the guide below. Try to identify all of the external and internal structures in bold. For each structure, know the basic function and the organ system to which it belongs.
2. For preserved specimens, conduct a thorough examination of the external structures. Then follow the dissection guide below to expose and identify internal structures. Be sure to complete each numbered step in sequence. Again, try to identify all of the structures in bold. For each structure, know the basic function and the organ system to which it belongs.
3. In each sample animal try to identify the major structures/specializations associated with each of the following organ systems:
digestive respiratory circulatory excretory reproductive
integumentary neural/sensory skeletomuscular
Note that in some of these organisms some of these systems simply do not exist - their functions are mediated entirely by diffusional exchange.
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 in and work with 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.
4. There are many useful on-line video guides for dissection and structure identification for most of these animals.
JELLYFISH (Aurelia - the moon jelly)
[Ph. Cnidaria, Cl. Medusozoa:Scyphozoa]
Place a plastomount of the moon jelly Aurelia under a dissecting microscope. Position it oral side up (so that the oral arms appear on top of the gonads). Find the mouth opening at the center of the umbrella. The umbrella is surrounded by outer (aboral) exumbrella and inner (oral) subumbrella cell layers, separated by the mesoglia “jelly”. The mouth is surrounded by four long oral feeding arms, each lined with short oral tentacles. Deep to the mouth the gastrovascular cavity (stomach) connects to a branching network of hollow radial canals which carry food particles out to ring canal near the outer rim of the umbrella. Find the four horseshoe-shaped gonads surrounding the mouth. Inspect the rim of the umbrella. Find the ring of marginal tentacles. Zoom in on a few of these tentacles to see that each has a continuous row of stinging cnidocyte cells. Finally, look carefully at the evenly spaced gaps in the ring of marginal tentacles. In each gap is a small, circular sensory cell. These may be photosensors, or statocycts, which help orient the jellyfish in the water.
PLANARIA (Dugesia)
[Ph. Platyhelminthes, Cl. Turbellaria]
Mount a slide of planaria on a compound microscope. One of the two mounted planaria is uniformly stained and the other has the intestine highlighted. Each organ system in the planaria is laterally paired, is highly branched, and extends the length of the animal. As the diagrams show, these include the nerve cords extending posteriorly from the brain, protonephridia (excretory system), yolk glands and oviducts (female reproductive systems) extending posteriorly from the ovaries, and the multiple testes (male reprouctive system) connected by sperm ducts.
Locate the light-sensitive eye-spots at the rostral end. The cephalic ganglion (“brain”) lies just deep to these. Locate the mouth, opening into the muscular pharynx near the midpoint of the body. Trace the blind-ended intestine (gut) through its two lateral caudal branches and it single midline rostral branch. Just caudal to the mouth you should be able to distinguish the seminal receptacle or copulatory sac and the penis.
ROUNDWORM (Ascaris lumbricoides)
[Ph. Nematoda]
Select an Ascaris, place in in a small dissecting tray, gently stretch it out, and carefully pin it at both ends. Determine whether your worm is a male (shorter, with a prominent caudal hook) or a female (longer, with no caudal hook). Using a sharp scalpel, carefully create a shallow slit the length of the worm. Split open the worm and locate the prominent intestine running the length of the animal, from the mouth and esophagus to the anus (female) or cloaca (male).
Deflect the intestine to one side. Locate the paired excretory tubes extending caudally from the small excretory pore near the mouth. In the female, find the genital pore, short vagina, and paired uterus horns with the oviducts and ovaries coiled around them. In the male find the single testis, vas deferens, and seminal vesicle which empties into the cloaca, a common caudal reproductive and digestive (anal) opening.
CLAM
[Ph. Mollusca, Cl. Bivalvia]
On the model of the clam “on a half shell” start by locating the two large adductor muscles which close the shells and the prominent muscular foot. Trace the digestive system from the mouth, past the stomach with its large digestive gland, through the torsioned (looped) gut, to the anus. Find the heart surrounding the gut near the anus. Locate the reproductive gonad near the center of the clam. Note that the mantle cavity surrounds the entire visceral mass and separates it from the secreted shell. Water flows from the incurrent siphon to the excurrent siphon and through the mantle cavity, passing over the prominent gills. The gills are involved in both respiratory gas exchange and filtering small suspended foot particles from the surrounding water.
EARTHWORM (Lumbricus terrestris)
[Ph. Annelida, Cl. Errantia:Oligochaeta]
Notice the prominent segmental organization of this animal. Locate the anterior or rostral end with its small mouth and the posterior or caudal end, bearing the rump or pygidium. Identify the dorsal and ventral surfaces. The dorsal surface will be darker and smoother. The ventral surface will be lighter and eight small bumps or setae on each segment. The setae can be felt by gently drawing the earthworm between your thumb and forefinger. The clitellum is a thickened sheath covering segments 32-37.
In a dissecting pan, place the worm with the darker dorsal surface uppermost and the setae downward. Stretch the animal to its natural full length, then pin it down at the extreme anterior and posterior ends. Starting at a point posterior to the clitellum and proceeding anteriorly, gently cut through the body wall only, along the mid–dorsal line. Keep your scissors flat and the cut shallow, so that you do not cut internal organs. Carefully separate the two sides and with a scalpel cut through the septa and membranes, which separate each segment internally. Pin out the body wall as you go. Finally extend your cut posteriorly through the posterior segments to the anus, again pinning out the body wall as you go.
The earthworm is an example of an animal with a complete digestive tract. Locate the mouth opening at the anterior end of the body. Locate the muscular pharynx, which leads to the esophagus. Find the bulbous soft crop and muscular gizzard. The crop stores food, sending it slowly to the thick, muscular gizzard where grinding occurs with the aid of sand particles. From here food passes to the long intestine, which runs posteriorly to the anus. Compare the structures visible on your dissected worm with the diagrams and the plastic model of the worm.
Stop for a moment and examine the prepared slide of the cross section of the earthworm under a compound microscope. Identify the body wall, the coelom and the intestinal wall. Note the infolding of the intestinal wall, the typhlosole, which increases absorptive surface area.
Annelids have relatively simple closed circulatory systems. Note the five pairs of aortic loops or arches ("hearts") located between the seventh and eleventh somites (body segments). These aortic loops encircle the digestive tract and connect the dorsal vessel with the ventral vessel. From these aortic loops blood is pumped posteriorly via the ventral vessel. From the ventral vessel blood flows out into smaller segmental vessels which branch throughout the tissues. The dorsal vessel collects blood and forces it anteriorly to the aortic loops for recirculation.
Annelids exchange respiratory gases through their external cuticles, or skins, so there are no unique internal respiratory structutres.
The excretory structures in the earthworm are paired nephridia in each segment. Try to locate these in your dissected worm under a dissecting microscope, using the earthworm model as a guide. These will be easier to locate in the segments posterior to the crop and gizzard. Locate the nephridia in the earthworm cross-section slide - they are found in a ventrolateral position in each segment.
Earthworms are hermaphroditic, having both complete male and female reproductive systems.
The ovaries are minute structures attached to the anterior septum (body partition) of segment 13. When the eggs are mature they rupture from the ovary and are released into the body cavity (coelom). The eggs pass to the funnel shaped openings of the oviduct which transfers them to the egg sac in the 14th segment. Eggs leave the oviduct through minute oviduct openings on the ventral side of the 14th segment.
Two pairs of minute testes are located in the 10th and 11th segments. Immature sperm are liberated from the testes into the large surrounding chamber called the seminal reservoir. It is in this reservoir and in the three pairs of seminal vesicles that the male gametes mature into sperm. When two earthworms come together during copulation, the sperm are passed by means of ciliated funnels into the long sperm ducts which pass posteriorly to the external sperm duct openings on the ventral side of segment 15. Therefore, copulation in earthworms represents mutual sperm exchange between two hermaphroditic organisms.
The remaining reproductive structures are the seminal receptacles which are two pairs of blind sacs found in segments 9 and 10. These seminal receptacles receive the sperm of the other worm during copulation, so they are part of the female reproductive system. The sperm of the other worm are stored in the seminal receptacles until the eggs are ready for release.
Earthworms have a cephalized nervous system with a circumesophageal brain and a solid ventral nerve cord. In your dissected worm the brain (superesophageal ganglion)may be found lying atop (dorsal to) the esophagus at the extreme rostral end of the animal. Gently pull the intestine to one side caudal to the clitellum and see if you can find the ventral nerve cord with its enlarged segmental ganglia.
CRAYFISH (Procambarus clarkii)
[Ph. Arthropoda, SuperCl. Crustacea, Cl. Decapoda]
Obtain a crayfish and place it dorsal side up in a dissecting pan. Identify the two body tagmata, the rostral cephalothorax and the caudal abdomen. The anterior end of the crayfish is the pointed rostrum and the posterior end is the tailfan, with its central telson and lateral uropods. Just behind the rostrum identify the long antennae and the shorter antennules, as well as the eyestalks.
Flip the crayfish over, so that it is ventral side up. At the rostral end, insert a blunt probe into the mouth, separating the heavy mandibles, two pairs of jointed maxillae, and three pairs of longer jointed maxillipeds. At the base of each antenna is the tiny nephidiopore opening which is the exit of the green gland ducts of the excretory (urinary) system. The crayfish has five pairs of “walking legs”, the first pair of which are the enlarged claws or chilapeds. Each abdominal segment has an additional pair of modified legs called swimmerettes. In the male crayfish the first pair are modified into more robust gonapodia for transferring sperm. Is your crayfish a male or female?
Flip the crayfish back over so that it is dorsal side up. Using a pair of scissors split the dorsal carapace down the midline, starting from the telson and continuing forward to just behind the rostrum. Keep the lower blade of the scissors parallel to the carapace and try to cut just through the carapace. Gently split the carapace away from the muscle of the abdomen. Entirely remove the carapace from each side of the cephalothorax, again being careful not to tear the underlying soft tissues and muscles.
In the cephalothrax locate the feathery lateral gills, each attached to the base of a walking leg. Pull the gills laterally to reveal the more medial cream-colored gonads of the reproductive system. These will be testes in the male and ovaries in the female. Located on the dorsal midline between the gonads is the tiny box-shaded heart. Crayfish have an open circulatory system, so the heart will not be attached to any obvious blood vessels. Rostral to the heart is the large, sack-like stomach, with cream-colored gastric glands on either side. Open the stomach to reveal the sets of internal teeth. Carefully pull on the stomach to completely remove it. Under the stomach you should find thin, white, paired neural connectives leading forward around the esophagus to the tiny, lobed brain. Under (ventral to) and slightly lateral to these connectives you should find the large green glands, the excretory glands or “kidneys” of the crayfish.
Return to the abdomen and locate the intestine running dorsal to the abdominal muscles. Trace the intestine to its termination at the anus, at the base of the telson. Finally, carefully strip the abdominal muscle mass away from the ventral cuticle of the abdomen. Carefully examine the ventral surface of this muscle mass and the exposed inner surface of the ventral cuticle. Attached to one of these you should find the thin, white ventral nerve cord with enlarged segmental ganglia.
STARFISH
[Ph. Echinodermata, Cl. Asteroidea]
Place a starfish in a dissection pan. On the oral (lower) surface locate the central mouth opening into the central stomach, the ambulacral canals running down the center of each of the five arms (rays), and the rows of tiny tube feet to either side. The stomach can be everted out through the mouth to digest prey animals that the starfish has captured. On the aboral (upper) surface identify the madreporite, off- center between the bases of two of the arms. The madreporite is the external opening of the water vascular system which the starfish uses to move. At the end of each arm locate a light-sensitive ocellus.
To dissect the starfish, start by flipping it over so that the oral surface is down. Choose an arm that is roughly opposite the madreporite. Using a pair of scissors clip off the end of the arm. Insert the tip of the scissors into the end of the arm and carefully cut through the cuticle down one side of the arm, separating the aboral and oral surfaces. When you reach the central hub, continue the cut around the circumference of the hub, creating a round flap from the aboral surface. Be sure to cut medial to the madreporite, so that it remains attached to the starfish and not to the flap. When you reach the dissected arm again, cut down the other side to the tip.
Carefully lift and detach the aboral cuticle from the central hub and the dissected arm. Carefully remove the soft, membranous stomach from the central hub, exposing the short stone canal leading from the madreporite to the circular ring canal of the water vascular system. In the dissected arm, carefully remove the sack-like gastric gland, which extends out into the arm under the aboral cuticle. This should reveal the radial canal running down the axis of the arm, with the attached ampullae (bulbs) of the tube feet on either side. To either side of the radial canal a lobe of the gonads also extends into each arm.
SEA LANCELET (Branchiostoma/Amphioxus)
[Ph. Chordata, SubPh. Cephalochordata]
Place a plastomount of the lancelet Amphioxus (Brachiostoma) under a dissecting microscope. Start by distinguishing the rostral and caudal ends and the dorsal and ventral surfaces. At the rostral end locate the mouth with its buccal cirri (short muscular tentacles). The branchial gill basket with its parallel gill slits surrounds the pharynx and functions both in respiratory exchange and in filter-feeding. Posterior to the pharynx the simple gut extends to the ventral anus. The gonads lie ventral to the gut. The cross-hatched structure running the length of the animal is the notochord. Dorsal to the notochord runs the hollow nerve cord. Finally, note that the herringbone pattern of the segmental muscles extends to the post-anal tail.