Echinoderms and Invertebrate Chordates
A. PHYLUM: ECHINODERMATA
Although it is evident that the ancestors of echinoderms were bilaterally symmetrical, as their larvae still are, the adult stages of these animals represent perhaps the most disjunct body form of any kind of animal. Whereas the radial symmetry of Cnidarians is typically based on the multiples of 2 or 3 (4, 6 or 8 part symmetry), echinoderms have selected 5 as the basic unit of adult symmetry (pentamerous symmetry). Other unique features of this phylum include 1) a water vascular system for operation of the tube feet, 2) lack of an excretory system, 3) highly reduced circulatory system, 4) presence of pedicellariae on the skin surface, and 5) skeleton (endoskeleton) composed of limey, rigid plates or movable spines and ossicles embedded in the skin.
The classes Asteriodea (seastars) and Ophiuroidea (brittle stars). have been combined by some zoologists under a new class called the Stelleroidea, based primarily on the similarity of their larval types, and are regarded now as subclasses. to avoid unnecessary confusion, however, We will use the older classification used in our text book.
1. Class: Asteriodea (seastars)
Obtain a preserved Asterias, a common atlantic coast seastar (the old term of starfish, which is still widely used, is being discouraged because the term "fish" in the name falsely implies some direct relationship to vertebrates), and rinse it off. Locate the external features shown in Fig. 25-2 of your text (p. 545). Note that the tube feet are technically called "podia." These podia are grouped in rows in grooves, the ambulacral grooves, along the length of each arm. Adult seastars do not have a head and therefore lack an anterior and posterior end. They also lack a dorsal or ventral surface. As in the Cnidaria (the other radial group we have studied) orientation is determined relative to the surface that has the tube feet and the mouth: this surface is called the oral surface. The surface which lacks the mouth and tube feet is called the aboral surface. On which surface is the madreporite located?
Turn the animal so that it is resting on its aboral surface. At the center of the arms where the ambulacral grooves meet is the mouth opening. In some specimens a thin-walled cardiac stomach may be everted through the mouth. This eversible part to the stomach is useful in slipping through the partly opened valves of a clam or oyster. The seastar can then digest the bivalve within its own shell!
Cut off one of the arms about 1-2 cm from, where it joins the main body and compare its cross-section with the features of Fig. 25-3C. If the gonads can not be found (they will be small if these seastars were collected in their non-reproductive period), don't worry; we can see them later in the dissection of the water vascular system. Read what the text (p. 454) says about the water vascular system.
Viewing the specimen under a dissecting microscope will enable you to search for the pedicellariae on the skin surface. You should be able to find, in addition to the madreporite, three aboral surface features: l) spines (large, white, rounded dermal ossicles), 2) dermal gills (soft, fleshy structures surrounding the spines), and 3) pedicellariae (smaller, white pincher-like structures near the gills or surrounding spines). Can you name a possible function for each of the 3 structures?
2. Dermal Gills
Remove the covering (epidermis and endoskeleton) from the aboral side of one arm, including the disk at the center of the arms. Be careful here, however, not to cut off the madreporite. Cut around this structure, leaving it in place. It is attached internally and will not fall off as the rest of the disk is removed. In the arms you will see the yellowish-green pyloric caeca. Remove these and look closely along the inner edges of the arm (under the pyloric caeca) to find the gonads, 2 per arm. The gonads are small and appear as tapered off-white to beige colored structures which have a definite puffy or bubbly appearance. Look at these under the dissecting microscope. During the breeding season these gonads swell with gametes and extend to nearly the tips of the arms. Return to the madreporite and follow the hard stone canal from the madreporite toward the oral surface where it joins the ring canal. In order to do this you will have to remove the pyloric stomach and the cardiac stomach. Asteroids have a complete digestive system. You may be able to see the intestine if it was not removed with the stomach. The anus used to be on the aboral surface that you just removed. Having removed the digestive system, now you can follow the stone canal to where it joins the ring canal. This circular ring canal surrounds the mouth and has several large radial canals branching off of it, one into each arm. You will not be able to directly see the radial canal, however. What you will see in each arm is the hard ambulacral ridges which are fused over the radial canal. In order to see the actual radial canal, you must cut off the tip of one arm and look for a small hole in the ambulacral ridge (go back to the earlier cross-section that you made). Look again at the ring canal which surrounds the mouth. Along the aboral surface of the ring canal, between the regions where the ambulacral ridges join the ring canal, search for small, fleshy bag-like structures. These are the polian vesicles which function as a reservoir for the fluids of the water-vascular system. Locate the ampullae, lateral canals and the tube feet in the arms.
The phylum Echinodermata includes starfishes or sea stars, brittle stars, sea urchins, sea lilies, and sea cucumbers. All but the last have a limy internal skeleton and hard external spines or plates. They are fixed or slow-moving inhabitants of the sea, from the high-tide zone to considerable depths. Often they are abundant but none form colonies. Species of shallow water are easily collected by hand at low tide and deeper ones are captured by dredging. Those with skeletons are easily prepared merely by drying but specimens for dissection are preserved in formalin or alcohol. Eggs of starfishes and sea urchins can readily be obtained in quantity and fertilized as needed; hence, they serve for study in embryonic development and in many experimental researches on animal eggs.
Common species of starfishes used for class work are Asterias forbesi and A. vulgaris of the Atlantic coast and Pisaster ochraceus of the Pacific coast.
PURPOSE: To study the internal and external anatomy of a starfish
MATERIALS: A preserved specimen, dissecting pan, scalpel or razor blade, probe, hand lens
Kingdom - Animalia Phylum - Echinodermata
1. EXTERNAL DISSECTION
A. Study a fluid-preserved specimen in a pan of water and identify:
1. Arms or rays - projecting from disc
2. Central disc - poorly defined
3. Oral surface - usually concave
4. Aboral surface - exposed in life
5. Madreporite - small white circular area, off-center on aboral surface of disc
6. Anus - minute, centered aborally on disc
7. Bivium - the two arms close to the madreporite aboral surface
9. Eyespot - small, pigmented on one end of each arm
10. Ambulacral grooves - one along oral surface of each ray
11. Ambulacral spines - slender rods on margins of ambulacral grooves
12. Tube feet - soft, slender, with expanded tips; 2 or4 rows in each groove
13. Tentacle - soft, on end of each arm
B. Examine a small area on the aboral surface under a binocular microscope and distinguish the following:
1. Papulae or dermal branchiae - thin hollow soft projections which function as gills
2. Pedicellariae - minute pincers with two jaws; in circles around spines and elsewhere
2. INTERNAL DISSECTION
With the starfish in water and the aboral surface uppermost, use stout scissors to cut off the extreme tip of each arm of the trivium. Then cut along the sides of these three arms. Use care not to injure any internal organs. In turn, lift and carefully remove the aboral surface of each arm, loosening the delicate mesenteries beneath by which the soft organs are attached. Also, cut around the disc (but not the bivium) and remove the aboral surface, leaving the madreporite in place. Finally, cut transversely, at mid-length, through one arm of the bivium to provide a cross section. Identify:
Coelom or body cavity - space containing internal organs; lined with thin ciliated peritoneum.
Stomach - disc, thin, sac-like, and 5-lobed, cardiac portion, larger, with pleated walls and retractor muscles; pyloric portion, aboral, smaller, 5-sided and smoother
Intestine - very slender, short, from pyloric stomach to anus
Hepatic caeca - a pair in each arm, greenish, long, of many finger-like lobes, each caecum with duct to pyloric stomach; also termer digestive glands, liver, or pyloric caeca.
Gonads - in each arm, below hepatic caeca, bilobed; each attached by duct opening aborally; sexes separate.
3. WATER VASCULAR SYSTEM
Remove the side of the stomach near the madreporite; then starting from the latter, trace the parts of the system. If
available, examine a demonstration specimen having the system injected with colored mass. Identify the following structures:
1. Stone canal - limy tube in an angle of bivium, from madreporite to ring canal.
2. Ring canal - hard, circular, around mouth region
3. Tiedemann bodies - nine, small swellings in ring canal
4. Radial canal - from ring canal along each arm, see cross section; connects by transverse canals to ampullae.
5. Ampullae - many, small, spherical, in floor of coelom -connect to tube feet
6. Tube feet
· What is the mode of action of the water vascular system?
· How do the ampullae and tube feet act to affect locomotion?
How do the tube feet serve in food taking? In adhering to solid objects?
2. Class Ophiuroidea (brittle stars)
The Ophiuroideans are commonly known as brittle stars because of their habit of easily breaking off their arms when handled. They are also known as serpent stars or basket stars. All members of this subclass are marine and live from the shallow subtidal areas to the deep sea plains. Most are free-living although a few are commensals with sponges and crinoids. This is probably the most successful subclass of echinoderm and has about 2,000 species.
Take a brittle star out of the jar and place it in a dissecting tray. Superficially it resembles an Asteroidean sea star in that it has an oral disk and arms. Notice the shape of the arms. Are they tapered like the sea star's? ____ Look at the aboral surface of the central disk. The madreporite, if present, is on the oral surface of the ophiuroids, but is often difficult to see. How does this compare to the location in the asteroids?
Observe the oral surface using a dissecting microscope. Look at the mouth region for the five slit-like openings. Along the edges of the opening are papillae or teeth. How would these function in feeding? Could it move these teeth? _____
In addition to the above characters, describe two additional features which you can observe that separate the ophiuroids from the asteroids:
2. Class Echinoidea (sand dollars and sea urchins)
dissected sea urchins will be available. Examine first the entire specimens for
external details. Urchins have been described as "sea stars which touched
their toes together over their head" because of the globular body form and
the fact that the tube feet occur in five areas starting on the oral surface
next to the mouth and converging on the aboral
surface next to the anus (on top of the globe). The most obvious thing you will
first see are the long spines covering the body. Note
their arrangement in rows. To get a better idea about this arrangement, look at
a broken peice of urchin shell (test). How do the
Living urchins move by "walking" on some of the spines like stilts, so muscles must be present somewhere. Pull off a spine and note the method of articulation to the shell. What shape do most of the body plates assume?_______________________ Fairly large, stalked pedicellariae will be visible over the entire surface.
Tube feet are located in the same belts as the spines, called the ambulacral areas, corresponding to the ambulacral grooves of the seastars. In live urchins, the tube feet can become extremely long and thin in order to reach out beyond the tips of the adjacent spines. The number, shape and arrangement of the body plates is extensively used in the classification of urchins and vary greatly from one group to another.
Look into the sea urchin (on demonstration) which has had its aboral end removed (compare to Fig. 25-12 in text). You should be able to see the gonads (similar looking in both sexes), the intestine running aborally toward the anus, and water vascular system. The water vascular system appears as five groups of two rows of brown membranous sacs with a tube running between them. Where these have been pulled from the inside of the test, you can see the two rows of holes that the tube feet passed through.
Looking down toward the oral half of the test you can see the centrally-located feeding apparatus called Aristotle's Lantern. This device, which is found only in sea urchins and sand dollars, is a structure of great complexity and beauty. It has a basic pentamerous symmetry and each of the five sides has 8 separate parts which are moved by 12 muscles (Figure 24-20). The entire complex has 40 separate pieces and 60 muscles! In a separate dish you can see one that is partially dissected. It is interesting to observe on a partially cleaned, dry piece, the intricacy of the fine lamellae for muscle attachments which hold the plates together. There is nothing remotely resembling this remarkable feeding structure in other echinoderm classes or in any other kind of animal! Note how fragile these are; handle with care.
3. Class Holothuroidea (sea cucumbers)
are rather atypical echinoderms in which the body is elongated. The usual fused
plates, which make up the endoskeleton or test of most echinoderms, are present
as unfused limey spicules
which are concealed in the body wall. The animals lie on one side rather than
"face-down" (on the oral surface) like the other classes. The tube
feet are still visible on the surface, and function in locomotion. Small ones
are able to climb up the wall of a marine aquarium by using their podia. Really
big ones probably could not lift their weight in this manner. The text does not
mention that some of the sea cucumbers are extensively collected in the South
Pacific for sale in
When attacked by predators, holothurians eviscerate themselves by ejecting all of their internal organs out through their anus. This is often sufficient distraction to permit an escape and later they can regenerate an entire new set of internal body organs!
B. PHYLUM CHORDATA
All members of the phylum Chordata share 4 characteristics at some time during their life cycle: l) a dorsal, hollow nerve cord which is derived from surface ectoderm, 2) a solid notochord derived from mesoderm, 3) pharyngeal gill slits, and 4) a post-anal tail.
1. Subphylum Urochordata (=Tunicata)
Preserved tunicates are on demonstration. Living individuals often form small colonies on piers, rocks, shells, etc. along the intertidal zone and can be seen at low tides. When touched, one will squirt some water from its grape-like body (hence a common name of sea-squirt). Most of the interior space is taken up with the large pharynx which is used as a filtering device (Fig. 26-5). The swimming larvae have all chordate characteristics, including the notochord. Most of these structures are lost as it develops into a sessile adult. Tunicates are obviously primitive chordates, but still retain all of the chordate characteristic...at least in their larval stages. As a subphylum, they appear to be a specialized side-branch of chordates and have not directly given rise to the other subphyla of the chordates.
2. Subphylum Cephalochordata
of the members of the small, exclusively marine group belong to a genus once
This was later renamed Branchiostoma but
we have kept the old name of amphioxus as kind of a "common name."
There are some adults on demonstration to show general size and shape. Even though there is no real "head", we have here a precursor of a typical vertebrate with the beginnings of many of our own organs. There is a small swelling at the anterior end of the nerve cord which begins a trend of increasing cephalization found in higher chordates. Since amphioxus is a filter feeder, the pharyngeal chamber , which contains the gill bars, is quite large, up to a third of the body length, and the stomach-intestine is short. The body musculature is serially arranged in bands (myotomes) suggesting a metameric origin and is similar to what occurs in fish.
Obtain a slide of a whole mount of a small specimen and locate as many of the structures labeled in Figure 26-8 as possible. Be sure to recognize the difference between the dorsal hollow nerve cord and the notochord. Which one of these two different structures is more dorsally located? ______________ You mat notice a "dotted" black line just above the notochord. This is a dorsal blood vessel.
You will see that the pharyngeal structure looks much different than it appears in the textbook drawing. How many gill bars do you count in your specimen? ___ If you were to drop an imaginary vertical line from the notochord to the ventral surface, how many gill bars would it cut through?
Prepare a labeled drawing (outline only) of an amphioxus across the top third of a page of your drawing paper.
Now, obtain a microscope slide labeled "Amphioxus-4 regions" or "Amphioxus-cross-section". This was made from adult specimens and, in addition to the structures seen on the whole mount of the juvenile, will show gonads which extend well forward into the pharyngeal atrium. Male gonads will appear to have a fine, dense structure. Female gonads will be composed of large, blocky units (ova) each with a large, rounded, clear nucleus. In cross-section, these will be seen along with the cross-sections of the gill bars.
After examining the four cross sections (refer to notes below) try to decide from which region of the whole body each section was cut, and draw an appropriate vertical line through the outline to indicate this. Now make a drawing from each of the sections on your slide in the appropriate space below the outline sketch. Try to make them to the correct scale.. If you are not certain that you have it right, have your instructor look at your results (before returning your cross-section slide)
Notes on the cross-section slide: The first section on the slide may show a semi-circle of dark spots which will be the cross-sections of the oral tentacles, or it may be slightly behind this- through the pharynx. Note that amphioxus does not have any jaws and hence can't close its mouth. The second section will be much larger and will show a complete ring of dark spots, which are the gill bars in section. The gonads should also be here, as well as a hollow structure which you should be able to identify. What is this hollow structure?
The 3rd section will show one or two sections of the gut (intestine and/or hepatic cecum) as well as two large dense masses of gonads on the ventral side.
Two features will occur on all four sections: The round- oval notochord and the much smaller squarish (or triangular) nerve cord which lies inside a chamber just on top of (dorsal to) the notochord. See if yu can find the small hole in the center of the dorsal nerve cord which makes it a hollow dorsal nerve cord. In all four sections, you will also see the body musculature. The obliquely vertical bands of muscle are called myotomes and separated by membranous partitions called myosepta
There is no distinct heart. The blood is pumped by general contractions of the larger vessels. It is pumped forward through the ventral aorta, up through the gill bars where oxygenation takes place (you can find these branchial arteries in cross- section 2), then posteriorly through the dorsal aortas (two in the gill region, merging into one farther back). This is basically similar to the ground-plan of circulation in fish.
These organisms set the stage for the vertebrates we will examine later in this class.
The Hemal System- The hemal system is made up of channels, all connected to each other by the axial sinus and working together to circulate nutrients to the rest of the body from the digestive tract. This involves the dorsal sac, which acts as a heart because it beats, thus helping the circulation. (picture located below)