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Phylum Arthropoda
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Phylum Echinodermata
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INTRODUCTION TO ARTHROPODS
With their segmented, metameric bodies with serial coelomic cavities, and their ventral nerve cord with circumpharyngeal connectives, the annelids reviewed in the last exercise set the structural stage for the emergence of an immense group of organisms called the arthropods. Traditionally, arthropods have been organized into a single phylum (Arthropoda) although some modern specialists recognize no less than three distinct phyla within this group. Our text strikes a kind of compromise in listing three subphyla (Chelicerata, Crustacea, and Uniramia) which correspond to the phyla defined by others.
One of the most striking advances made by the arthropods over the annelids is the development of segmented, moveable appendages which are developed from the body wall. As we will see in this exercise, appendages have been adapted and modified by arthropods in an astonishing variety of ways: as sensory organs; as a multitude of different mouth parts; for walking, crawling, climbing, jumping, and swimming; for sperm transfer; for egg care; for defense. All, no matter how complex or exotic, can be shown to be homologous derivatives of simple unspecialized appendages.
Although crustaceans and chelicerates probably originated as marine animals before the appearance of tracheates, we will reverse the order of studying them to better observe the striking similarities between generalized annelids (like Nereis) and generalized tracheates (like centipedes). There is no reason to believe that the latter group originated from the former, but it is highly probable that both had a common ancestor.
A. Subphylum Uniramia
This phylum contains classes that are primarily terrestrial (except for a few insects which live in freshwater) and are characterized by having tracheae and mandibles. In this exercise we will omit the small classes Symphyla and Pauropoda, and focus on the centipedes, millipedes, and insects, beginning with the least specialized tracheates, the Chilopoda. Although the centipedes (Chilopoda) and millipedes (Diplopoda) appear very similar at first glance, there are many difference between them. As you go through this lab exercise, make a note of these differences.
1. Class Chilopoda
Preserved specimens of several orders of Chilopoda will be available for study. Please handle them with care to avoid undue breakage of legs. Note that the elongated body is composed of numerous similar segments, most of which have a single pair of legs. The last pair is often larger and longer and may be used for defense, for food capture, or for mating. The first pair is turned forward beneath the small head capsule and modified into a pair of fangs equipped with poison glands. These modified legs (see maxillipeds, Fig. 21-32) are called the prehensors. No other known animal has modified its walking legs into poison fangs!
Notice that the head capsule is strongly flattened, with only a few eyespots (ocelli) on each side (many centipedes have none). The antennae contain a large number of small, similar segments (antennomeres), the number usually constant for a given species. They are provided with numerous sensory hairs and taste buds. The mouth parts are small and centered on the underside of the head, and are composed of three pairs of appendages: mandibles, 1st maxillae, and 2nd maxillae, which we will not dissect out. The enormous prehensors are the first modified pair of legs, and you can see their attachment to the first body segment. Also notice that the individual prehensor is segmented. How many units can you count? ______ How does this compare with the number of leg segments? _________
Looking along the sides of the body, notice that the legs are attached to the small sclerites (pleurites) which lie in the membrane between the large dorsal plates (terga) and ventral plates (sterna). On some segments you can find the stigmal openings above the leg bases; these are openings into the tracheal system which functions in gas exchange and delivers air into every part of the body. Which segments have stigmata? ___________
Notice that even though the body sclerites are pretty tough, they are flexible. Push one softly with the point of a pin. Does it dent? ______ The body of the average centipede is rather elastic, and can be stretched and contracted almost like a worm.
2. Class Diplopoda
Millipedes used to be classified with the centipedes under the collective group name of "Myriapoda" which means "many legs." Actually the two classes are only remotely related within the Uniramia. Millipeds are much more diversified structurally than centipedes and are far more numerous.
Most millipedes are basically cylindrical in cross-section. How does this compare to the shape of centipedes? Millipede body segments are actually composed of two embryonic somites fused together during development. The result is that each apparent metamere is actually a double one, having two pairs of legs. Another feature of millipeds is that the body wall is heavily infused with calcium carbonate, making it rigid and brittle. Push on it a little like you did on the centipede. Is it as pliable?
In most millipeds the two original somites of each "segment" can be distinguished, the anterior one (prozonum) being smaller in diameter and telescoped into the somewhat larger posterior unit (metazonum) of the preceding segment.
The head capsule, like that of centipedes, is composed of 6 embryonic somites. Eyes or ocelli are absent from many species. The antennae are also different from centipedes. How many antennomeres does your millipede have? _________
The first big segment behind the head, called the collum, has no legs. The next three body segments have only a single pair of legs as a result of the normal second pair of legs having been lost from each segment. Our text book refers to this as the "thorax", but that is not correct. This is thought to be an adaptation for rolling up as it would make more room for the head to curl around underneath. Some millipeds can roll up into a perfect sphere.
Millipedes characteristically have pores along the sides of their bodies through which a volatile fluid is released from repugnatorial glands, when the animal is threatened. This fluid is not toxic, but may be aromatic and has a bitter taste. The fluid also serves as a fungicide, useful for animals that live in damp places.
On the underside, you will see that the two pairs of legs originate on an elevated platform. The anterior pair of legs really belongs to the prozonum, but had to be moved back in order for the prozonum to be telescoped into the metazonum (like a ball and socket joint). How many podomeres are there in the legs? ______ How do these legs compare to centipede legs in terms of relative length and placement? Which arrangement do you think provides for faster movement?
The openings to the trachea (stigmata) also originate on the sterna, as tiny slits just above the bases of the legs, but these may be difficult to see.
Now you should be able to go back through this account and pick out at least eight major external differences between millipedes and centipedes.
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3. Class Insecta
The largest group of organisms on earth, now estimated at perhaps 50 million species, are dispersed among as many as 23 orders. We will look at the anatomy of a fairly "generalized" insect which retains many "primitive" characters. Figures 23-35 and 23-40 will be useful in the identification of the parts mentioned in this section.
Obtain a preserved grasshopper (Romalea), wash it off, and check out the external features as labeled in Fig. 23-35. Note the three major tagmata (divisions) of the body. Recall that the head capsule is actually a composite structure made up of six embryonic somites (a hint of this is given by the fact that each of the three pairs of appendages modified as mouthparts belongs to a separate somite). The prothorax has no wings, and is loosely attached to the pterothorax (which is the combined unit of the mesothorax and the metathorax). These two units are fused to give more rigidity for the leg and wing muscles. Several grooves crossing the prothorax are secondary infoldings to produce small phragmata on the inside, and do not indicate segmental fusion.
Lay the grasshopper on its side and look at its head. At the anterior end, you will see the 2 antennae with the large eyes bulging on the sides of the head. Look at the surface of the eye and see that it is a true compound eye. Crank up the power on the scope to its greatest magnification and look at the eye! How many images could the grasshopper see with each compound eye? Each facet of the compound eye is called an ommatidium. Are there other eyes on the grasshopper head? Look at these with your high power. Are these compound eyes or a large, single ocellus?
Switch back to the lowest magnification and find the "forehead" between the compound eyes. This large immovable plate is the frons. Two flexible plates just below the frons form the "upper lip." One is immovable and called the clypeus. (Consult Figure 23-40 for help with these mouth parts) The other movable piece of the "upper lip" is called the labrum. Notice that the junction between the clypeus and the labrum is flexible. This allows the labrum to move up and down like a flap. Now turn the grasshopper on its back. Notice the 2 small leg-like appendages near the bottom of the labrum. These modified leg appendages are maxillary palps and are 1 part of the 3 parts of the maxillae. To see the mandibles, lift up the labrum (upper lip). The mandibles lie just under the labrum and are about as big as it. Carefully tease the mandibles apart with your pointer to see how they work. You should be able to find each mandible with its 7 or so "teeth." Each species has a different number or location of bumps or "teeth" on its mandibles. Below this,you will find the lower lip or labium with its antennae-like labial palps.
The mandibles are used for grinding food and are characteristics of herbivores. what might be the function of the other mouth parts:
labrum? _____________________________________
maxillae? ____________________________________
labium? ______________________________________
With all of these mouth parts, do you think that a grasshopper could bite us? They potential can, but rarely do. How many grasshoppers have you caught as a kid and never been bitten? The local green Katydid however, is a different matter. Katydids are in the same Order (Orthoptera) as the grasshoppers ...but beware. Katydids can bite and you have to be careful how you hold them. Grab them from above and behind. They can not sting from the tip of the abdomen (like bees, wasps, and some ants) but they can give a nasty nip with the mandibles. The next time you see a Katydid, check out the mouth parts!
The legs are composed of 7 apparent segments (podomeres). The one next to the body, the coxa, is actually a composite and rather complicated on the front side. Note the intricate jointing mechanism which approaches the ball-and-socket condition. The smaller trochanter is attached immovably to the femur, this by a hinge joint to the more slender tibia. Beyond this is the 3-jointed tarsus. The first tarsal unit has six little pads beneath, the 2nd has 2, the 3rd only one, but also a special pad between the claws. These various pads are useful in climbing on plant stems. The 3rd pair of legs is different. The femur is much larger to accommodate the jumping muscles, and the trochanter is fused to the coxa for greater strength.
Examine the abdominal region. How many segments are in the abdominal region of your grasshopper? Each segment has a spiracle (opening to the tracheal system) and the abdomen can be expanded and contracted to aid in the circulation of air.
At the rear end of the abdomen, the female will have four large pointed ovipositors which are used to drill a rather deep hole in sandy soil to accommodate the eggs. The tips are held together, pushed in, and then spread apart. This is repeated over and over until the abdomen is nearly out of sight. Then the eggs are laid. The male abdomen ends in a small cup-like structure, the genital capsule, with a triangular tergum on the top side and closed on the rear by a transverse sclerite with two small points on its top edge. On the ventral side there will be a very complicated set of male genitalia, the details of which differ for each species of grasshopper. Sometimes these differences are the only tangible differences between closely related species.
Check the counter for additional insects which are on demonstration, and note the demonstrations of gradual and complete metamorphosis. What might be the relative advantages and disadvantages (to the insects) of these two developmental plans?
B. Subphylum Crustacea
It will be recalled from the previous section that the definitive mouthpart in tracheate arthropods (Uniramia) are the mandible, supported by two smaller appendages, the 1st and 2nd maxillae. This arrangement is similar in crustaceans types as well, except that crustaceans may have additional mouthparts called maxillipeds. Crustaceans and uniramians collectively are often called mandibulate arthropods, and in both groups the mandibles are derived from appendages of the 2nd somite of the head.
Crustaceans differ, however, in several ways, such as having two pairs of antennae, gills instead of trachea, and appendages which are basically biramous (two-branched) instead of uniramous (from which the subphylum name "Uniramia" is derived). Some recent specialists think that these two kinds of mandibulates originated from different ancestors and should be placed in different phyla.
Crustaceans are enormously variable in structure. It should be remembered that in addition to the familiar crayfish, crabs and lobsters crustaceans come as isopods, amphipods, ostracods, cladocerans, copepods and barnacles. Many of these forms are microscopic and may not even be recognizable to the beginner as crustaceans. One advantage of studying a large crustacean is the ease with which the principle of modification of serially homologous appendages can be observed.
A. Order Decapoda ("ten-legged crustaceans"): crayfish, lobsters, shrimp, crabs.
Figures 23-13 and 23-16 will help you locate the following parts, but do follow along in the lab exercise for guidance and additional information. Obtain a preserved crayfish (Cambarus) and rinse it with water. There is no distinct head capsule as in insects and some other crustaceans. In the crayfish the anterior-most segments are all fused together with those of the thorax. The resulting body unit is called the cephalothorax and is covered by a hard exoskeleton called the carapace.
Behind this is the abdomen composed of six moveable segments and a thin flat posterior plate (the telson). Note that the carapace is divided into two regions by distinct groove (cephalic groove) about midway. Some authorities identify the region anterior to this groove as the head and that region posterior to the groove as the thorax. The rostrum extends over and between the eyes as a long sharply pointed projection. Look at the shape of the rostrum. Is it smooth edged or serrated?________ Is it long or stubby? Different species have differently shaped rostra.
The rear part of the carapace is usually roughened (warty) and there is a mid-dorsal hourglass shaped suture. This part of the carapace covers the gills and therefore makes up the outer wall of the branchiostegel chamber. Notice that the tergal parts of the abdomen overlap even when the abdomen is curled downward, whereas the sternal elements are reduced to a thin transverse strip. Why would there be such an arrangement (how would this area function)?
The underside of the cephalothorax has a single sternal structure running from front to back between the bases of the legs. This provides the median point of attachment (pivot) for the leg bases and usually can not be seen until the legs are removed.
Appendages:
The structure of the crustacean leg is highly complicated and varies so much from one group to the another that we will not go into details. What is more important for us to know is that the basic biramous appendage can be modified into an amazing variety of structures serving various purposes. From front to back we can recognize
1. Head appendages (5 pair)
a. Two pairs of antennae (sensory function)
b. One pair of mandibles (chewing)
c. Two pairs of maxillae (food processing)
2. Thoracic appendages (8 pair)
a. Three pairs of maxillipeds (food processing)
b. Five pairs of pereiopods (walking legs)
3. Abdominal appendages (6 pair)
a. Six pairs of pleopods (swimmerettes)
In males, the 1st two pleopods are modified for sperm transfer; the others are small except for the last pair which are broad and paddle-like (uropods) and are used for swimming. In females the pleopods are used for carrying eggs).
These all originated from similar appendage buds in embryonic somites. As you go along, notice how much each kind of appendage has been altered from the basic pleopod type and how appropriate this modification is for its function.
Using your fingers, remove the carapace from one side. Removal of the carapace will expose the gills, which are attached to the leg bases. Working under the microscope and with a pin, you can dislocate the legs on the same side, starting at the back (5th pereiopod) and freeing the leg where it attaches to the sternum. Tear away the intersegmental membrane so the entire leg with its attached gill comes free. Continue this process forward until all the legs are free. Notice the intricate system of knobs and interlocking bumps which make up the pivots on which the legs hinge. Some of the pereiopods are chelate, meaning that the next to the last segment extends forward under the last, making a kind of pincher. This chelate condition is most extreme on the first pair which are called chelipeds. Are all of the pereiopods chelate?
With the 1st pereiopod removed, hold it firmly in your fingers and flex it to see how the joints are placed: usually each joint (articulation) is at right angles to the next, making it possible for the animal to reach in all directions.
Now, in front of the bases of the 1st pereiopod you will find that the appendages suddenly get a lot smaller and, going forward, become almost membranous. At this point you may want to refer to Fig. 23-14 . Note that each mouth part consists of several parts. Proceed carefully, isolate and remove, in order, the 3rd, 2nd and 1st maxillipeds. Then remove the very small 2nd and 1st maxillae. You can get an idea of the biramous condition from these small appendages. The 2nd maxilla has a flat thin lobe on its outer side that is used to circulate water forward through the gill chamber. When feeding, a crayfish uses nearly all of the mouthparts; the smaller ones scoop up any bits and fragments that might be torn loose by the larger ones.
The mandibles may be left in place. They are large, heavy structures that are hinged at two places. Try moving it back and forth and search for these two pivot points. The mandible also has a large palp. In between the mandibles you can see the mouth.
Two more appendages remain to be examined: the larger 2nd antenna and the smaller 1st antenna (antennule). The 2nd antenna looks very leg-like at its base, with two large segments. There is also a large flattened antennal scale which has a sharp tip. What might this flattened scale be used for as the crayfish moves through the water?
What other differences between the antenna and a typical leg can you find?
At the base of each 2nd antenna is a small triangular with an oval opening centered on it. The hole is the opening of an excretory gland located inside the head (a sort of kidney).
Lastly, examine one of the stalked eyes. The eye is not an appendage even though it looks and moves like one. Some crabs have eye stalks two or three times as long as what we see here. The small brain is located between the eyes, but is difficult to observe.
Now, going back to the gill chamber, note that the inner wall is very thin and transparent. You may see some red streaks running downward toward where the legs used to be. The red color is a latex material injected into the heart (cardiac) chamber (more on this later). Just in front of where the first gills were, you will see a large mass of soft, spongy, white (or yellow) material. This is a digestive gland, sometimes called the hepatopancreas or "liver". Carefully remove as much of it as you can reach with a pin. Just dorsal to the gland is a large, brown, oval mass which was attached to the inside of the carapace you removed. This is the mandibular adductor muscle and with the gland removed you can see how it runs into a long tendon extending down to the mandibular base.
If you will now carefully break off the rostrum that extends forward, you will be able to see a large, almost-clear walled membranous bag which is called the cardiac stomach. There are some muscles on its upper side that run forward into the rostrum, you can pick these away.
The stomach has two distinct transverse stiffening structures on its dorsal side: an large anterior one and a smaller posterior one. If you will use a pin to tear open the stomach just behind the smaller posterior one, you will find inside some hard, black objects, the gastric mill, used to grind up the food before it passes into the next part of the stomach. This next part of the stomach is called the pyloric stomach and gradually merges into the intestine, which then extends on back through the abdominal region near the top side in a groove of the muscles just under the tergal plates. This is the dark line you often see in peeled shrimp. The process of removing this intestine in shrimp produces "de-veined" shrimp.
The big, red rubbery mass behind the stomach is latex used to color the circulatory system. The small, rectangular shapped heart is often imbedded in this mass of latex. If you have a male, the two small thumb-like testes may be found on top of the intestine just as it leaves the rear of the stomach.
At this point you could remove the carapace from the other side; this will allow room for you to look directly down into the body cavity. You can see two large brown masses under the stomach-intestinal region: these are the ventral longitudinal muscles and just above them are much thinner muscles associated with the leg bases. By pulling the sides of the body apart, you can look down in between the longitudinals and see the ventral nerve cord under them.
So much for the interior. Now on to the abdomen. If you have a male, the first two pleopods will be modified for sperm transfer, the first in particular which is heavily sclerotized. The end varies considerably and each species has a different shape. Even genera are separated on this basis. If you are interested, a reference can be consulted which will enable you to identify your specimen. If you have a female, note that all of the pleopods are similar in size and shape. Make sure you see both sexes of crayfish and can identify each. How many abdominal pleopods does your specimen have?________
Do male and female crayfish have different numbers of pleopods? ___________
In the female, notice that there is a small oval structure, the annulus ventralis, centered between the last pair of pereiopods. This serves as a seminal receptacle to hold the sperm packet which is placed there by the male during copulation. The ends of the oviducts open on the bases of the 3rd pair of pereiopods as a large oval spot surrounded by a fringe of hairs.
There is not much to see in the abdomen except the huge flexor muscles (crayfish swim backwards by quickly flexing the abdomen forward and under them, hence the muscle's name). You will need to remove the dorsal plate (tergum) from an abdominal segment to see this muscle. You will also see the intestine running down the center of this muscle. You can go back about halfway and make a complete cross-section through the abdomen to see these muscles which have a sort of "butterfly" outline. At the top is a small depression through which the gut and dorsal artery run. In a similar place on the ventral side you can find the ventral nerve. From what you have seen so far it should be obvious why, in crayfish and shrimp, only the abdominal muscles are eaten.
Other kinds of crustaceans are on demonstration. Compare their external anatomies with that of the non-reflexed crayfish.
C. Subphylum Chelicerata
The chelicerates are characterized by having fang-like chelicerae (lst pair of appendages), pedipalps (2nd pair of appendages), and 4 pairs of walking legs. The only exception to this generalized rule is the class Merostomata which has chelicerae, lacks pedipalps but has 5 pairs of walking legs. Chelicerates are quite different from Tracheates or Crustaceans. In one respect, they all lack antenna. Chelicerates are very abundant and include the familiar spiders,
scorpions, ticks, mites, daddy-long-legs (which are not true spiders), horse-shoe crabs and probably the ancient, now extinct trilobites. None of these arthropods have mandibles. In the chelicerates, the first appendage, the chelicera originates from the same embryonic somite that produces the antennae in mandibulates. The other anterior somites produce legs, not mouthparts. Sometimes the bases of the anterior-most legs do a little crushing or grinding of food, but in general the usual feeding technique of chelicerates is a kind of extra-oral digestion. The food object is killed (usually) and held next to the mouth with the pedipalps and chelicerae. Digestive enzymes are then pumped out of the mouth and into the prey, digesting the prey external to the chelicerate. The prey is reduced to a liquified soup and is then sucked in by the pumping action of the pharynx.
1. Class Merostomata
The horseshoe crab, Limulus is on demonstration: observe one carefully. You will see that a tough, leathery covering, the carapace covers the cephalothorax (fused head and thorax regions). Behind the horse-shoe-shaped cephalothorax is a smaller, tapering abdomen and a long posterior telson. Compare this telson with the telson on the crayfish. On the dorsal side, you will see two compound eyes and two simple eyes located on either side of a small anterior spine. Along the sides of the abdomen are a series of movable spines. On the ventral side you will see 6 pairs of appendages: the first, a pair of chelicerae, followed by four pairs of chelate legs (each with a pincher) and a longer specialized pair of appendages used for cleaning the gills and pushing the horseshoe crab through the sand. All members of this class are marine.
2. Class Pycnogonida
The "sea spiders" are strange looking animals which appear to be all legs. A number of specimens are on demonstration in jars. Examine several specimens and count the 4 pairs of large walking legs by locating their placement on the body. Anteriorly, sea spiders have a long proboscis, chelicerae and a smaller pair of legs used in bearing the eggs. All pycnogonids are marine, being found from temperate waters to arctic and antarctic waters. They are also found in some of the deepest depths of the oceans. Larger ones are usually found in colder waters. They are carnivores and smaller ones can be found feeding on marine hydroids such as Obelia.
3. Class Arachnida
This large class of chelicerate arthropods includes the spiders, scorpions, ticks, mites, harvestman (daddy long legs) and others. Various examples are on demonstration.
Observe the male and female spider. Can you tell which is which?_______ Contrary to popular belief, the female is not always larger than the male spider.
Look at the first pair of appendages on both specimens. Can you see the fangs on the chelicerae? All spiders, except one family, have internal poison glands which empty through the hollow fangs.
In males, the 2nd appendage (pedipalp) differs from that in females. The male's pedipalps are enlarged and modified for transferring sperm. They often resemble a pair of boxing gloves held in front of the spider. In female spiders, the pedipalp tips are rounded and are the same diameter as the rest of the appendage (i.e. there is no swollen bulb on the tip). Note that in both specimens the cephalothorax and abdomen are joined by a narrow waist-like pedicle. Mites and ticks have body regions fused so that there is no separation between the cephalothorax and abdomen. How many total appendages do arachnids have? _________
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. This classification differs from the one used in our text book. We will observe external anatomy of the sea star and brittle star.
1. Class: Stelleroidea
a. Seastars (subclass Asteroidea)
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!
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?
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b. Brittle stars (Subclass Ophiuroidea)
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 a 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?
In addition to the above characters, describe two additional features which you can observe that separate the ophiuroids from the asteroids:
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2. Class Echinoidea (sand dollars and sea urchins)
Several 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 spines attach?.
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 25-13). 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)
Holothurians (Fig 25-16) 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 Japan, where they are much esteemed as an item of food and are reputed to be an aphrodisiac along with numerous other exotic animal parts.
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!
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 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.