CHAPTER V

ECHINODERMS (SPINY ANIMALS)

THE STARFISH

SUGGESTIONS. Since the echinoderms are aberrant though interesting forms not in the regular line of development of animals, this chapter may be omitted if it is desired to shorten the course.--The common starfish occurs along the Atlantic coast. It is captured by wading along the shore when the tide is out. It is killed by immersion in warm, fresh water. Specimens are usually preserved in 4 per cent formalin. Dried starfish and sea urchins are also useful. A living starfish kept in a pail of salt water will be instructive.

[Illustration: FIG. 52.--Starfish on a rocky shore.]

[Illustration: FIG. 53.--PLAN of starfish; III, madreporite.]

=External Features.=--Starfish are usually brown or yellow. Why? (See Fig. 52.) Has it a head or tail? Right and left sides? What is the shape of the _disk_, or part which bears the five arms or _rays_? (Fig. 53.) Does the body as a whole have symmetry on two sides of a line (bilateral symmetry), or around a point (radial symmetry)? Do the separate rays have bilateral symmetry? The _skeleton_ consists of limy plates embedded in the tough skin (Fig. 54). Is the _skin_ rough or smooth? Hard or soft? Are the projections (or _spines_) in the skin long or short? The skin is hardened by the limy plates, except around the _mouth_, which is at the center of the lower side and surrounded by a membrane. Which is rougher, the mouth side, (_oral_ side) or the opposite (_aboral_ side)? Which side is more nearly flat? The _vent_ is at or near the center of the disk on the aboral surface. It is usually very small and sometimes absent. Why a vent is not of much use will be understood after learning how the starfish takes food.

[Illustration: FIG. 54.--LIMY PLATES in portion of a ray.]

[Illustration: FIG. 55.--Starfish (showing MADREPORITE).]

An organ peculiar to animals of this branch, and called the _madreporic plate_, or _madreporite_, is found on the aboral surface between the bases of two rays (Fig. 55). It is wartlike, and usually white or red. This plate is a _sieve_; the small openings keep out sand but allow water to filter through.

[Illustration: FIG. 56.--WATER tube System of starfish.

_m_, madreporite; _stc_, stone canal; _ap_, ampulla.]

=Movements: the Water-tube System.=--The water, which is filtered through the perforated madreporite, is needed to supply a _system of canals_ (Fig. 56). The madreporite opens into a canal called the _stone canal_, the wall of which is hardened by the same kind of material as that found in the skin. The stone canal leads to the ring canal which surrounds the mouth (Fig. 56). The ring canal sends _radial canals_ into each ray to supply the double row of _tube feet_ found in the groove at the lower side of each ray (Fig. 57). Because of their arrangement in rows, the feet are also called _ambulacral_ feet (Latin _ambulacra_, “forest walks”). There is a water holder (_ampulla_), or muscular water bulb at the base of each tube foot (Fig. 58). These contract and force the water into the tube feet and extend them. The cuplike ends of the tubes cling to the ground by suction. The feet contain delicate muscles by which they contract and shorten. Thus the animal pulls itself slowly along, hundreds of feet acting together. The tube feet, for their own protection, may contract and retire into the groove, the water which extended them being sent back into the ampulla. This system of water vessels (or water-vascular system) of the echinodermata is characteristic of them; _i.e._ is not found elsewhere in the animal kingdom. The grooves and the plates on each side of them occupy the _ambulacral areas_. The rows of spines on each side of the grooves are freely movable. (What advantage?) The spines on the aboral surface are not freely movable.

[Illustration: FIG. 57.--Starfish, from below; tube feet extended.]

[Illustration: FIG. 58.--SECTION OF ONE RAY and central portion of starfish.

_f1_, _f2_, _f3_, tube feet more or less extended; _au_, eye spot; _k_, gills; _da_, stomach; _m_, madreporite; _st_, stone canal; _p_, ampulla; _ei_, ovary.]

=Respiration.=--The _system of water vessels serves the additional purpose_ of bringing water containing oxygen into contact with various parts of the body, and the starfish was formerly thought to have no special respiratory organs. However there are holes in the aboral wall through which the folds of the delicate lining membrane protrude. These are now supposed to be _gills_ (_k_, Fig. 58).

=The nervous system= is so close to the aboral surface that much of it is visible without dissection. Its chief parts are a _nerve ring_ around the mouth, which sends off a _branch_ along each ray. These branches may be seen by separating the rows of tube feet. They end in a pigmented cell at the end of each ray called the _eye-spot_.

=The food= of starfish consists of such animals as crabs, snails, and oysters. When the prey is too large to be taken into the mouth, the starfish _turns its stomach inside out_ over the prey (Fig. 59). After the shells separate, the stomach is applied to the soft digestible parts. After the animal is eaten, the stomach is retracted. This odd way of eating is very economical to its digestive powers, for _only that part of the food which can be digested and absorbed is taken into the body_. Only the lower part of the stomach is wide and extensible. The upper portion (next to the aboral surface) is not so wide. This portion receives the secretion from five pairs of digestive glands, a pair of which is situated in each ray. Jaws and teeth are absent. (Why?) The vent is sometimes wanting. Why?

[Illustration: FIG. 59.--Starfish eating a sea snail.

_b_, stomach everted.]

=Reproduction.=--There is a pair of ovaries at the base of each ray of the female starfish (Fig. 58). The spermaries of the male have the same position and form as the ovaries, but they are lighter colored, usually white.[2]

[2] The sperm cells and egg cells are poured out into the water by the adults, and the sperm cell, which, like all sperm cells, has a vibratory, tail-like flagellum to propel it, reaches and fertilizes the egg cell.

=Regeneration after Mutilation.=--If a starfish loses one or more rays, they are replaced by growth. Only a very ignorant oyster-man, angry at the depredations of starfish upon his oyster beds, would chop starfish to pieces, as this only serves to multiply them. This power simulates multiplication by division in the simplest animals.

=Steps in Advance of Lower Branches.=--The starfish and other echinodermata have a more developed nervous system, sensory organs, and digestion, than forms previously studied; most distinctive of all, they have a body cavity distinct from the food cavity. The digestive glands, reproductive glands, and the fluid which serves imperfectly for blood, are in the body cavity. There is no heart or blood vessels. The motions of the stomach and the bending of the rays give motion to this fluid in the body cavity. It cannot be called blood, but it contains white blood corpuscles.

[Illustration: FIG. 60.--Young starfish crawling upon their mother. (Challenger Reports.)]

The starfish when first hatched is an actively swimming bilateral animal, but it soon becomes starlike (Fig. 60). The limy plates of the starfish belong neither to the outer nor inner layer (endoderm and ectoderm) of the body wall, but to a third or middle layer (mesoderm); for echinoderms, like the polyps, belong to the three-layered animals. In this its skeleton differs from the shell of a crawfish, which is formed by the hardening of the skin itself.

=Protective Coloration.=--Starfish are brown or yellow. This makes them inconspicuous on the brown rocks or yellow sands of the seashore. This is an example of protective coloration.

THE SEA URCHIN

=External Features.=--What is the _shape_ of the body? What kind of _symmetry_ has it? Do you find the oral (or mouth) surface? The aboral surface? Where is the body flattened? What is the shape of the spines? What is their use? How are the tube feet arranged? Where do the rows begin and end? Would you think a sea urchin placed upside down in water, could right itself less or more readily than a starfish? What advantage in turning would each have that the other would not have? The name sea urchin has no reference to a mischievous boy, but means sea hedgehog (French _oursin_, hedgehog), the name being suggested by its spines.

=Comparison of Starfish and Sea Urchin.=--The water system of the sea urchin, consisting of madreporite, tubes, and water bulbs, or ampullæ, is similar to that of the starfish. The tube feet and locomotion are alike. There is no need for well-developed respiratory organs in either animal, as the whole body, inside and out, is bathed in water. The method of reproduction is the same.

[Illustration: FIG. 61.--A SEA URCHIN crawling up the glass front wall of an aquarium (showing mouth spines and tube feet).]

[Illustration: FIG. 62.--A SEA URCHIN with spines removed, the limy plates showing the knobs on which the spines grew.]

[Illustration: FIG. 63.--SECTION OF SEA URCHIN with soft parts removed, showing the jaws which bear the teeth protruding in Fig. 62.]

The starfish eats animal food. The food of the sea urchin is almost exclusively vegetable, hence it needs teeth (Fig. 62, 63); its food tube is longer than that of a starfish, just as the food tube of a sheep, whose food digests slowly, is much longer than that of a dog.

[Illustration: FIG. 64.--THE SEA OTTER, an urchin with mouth (_o_) and vent (_A_) on same side of body.]

The largest species of sea urchins are almost as big as a child’s head, but this size is unusual. The spines are mounted on knobs, and the joint resembles a ball-and-socket joint, and allow as wide range of movement. Some sea urchins live on sandy shores, other species live upon the rocks. The sand dollars are lighter colored. (Why?) They are usually flatter and have lighter, thinner walls, for there is danger of sinking into the sand. The five-holed sand cake or sand dollar has its weight still further diminished by the holes, which also allow it to rise more easily through the water. The flattened lower surface of both starfish and sea urchin causes the body to remain still while the tube feet are stretching forward for another step.

OTHER ECHINODERMS

The =sea cucumbers=, or =holothurians=, resemble the sea urchin in many respects, but their bodies are elongated, and the limy plates are absent or very minute. The mouth is surrounded by tentacles (Fig. 65).

[Illustration: FIG. 65.--SEA CUCUMBERS.]

[Illustration: FIG. 66.--A BRITTLE STAR.]

The =brittle stars= resemble the starfish in form, but their rays are very slender, more distinct from the disk, and the tube feet are on the edges of the rays, not under them (Fig. 66).

The =crinoids= are the most ancient of the echinoderms. (Figs. 67, 68.) Their fossils are very abundant in the rocks. They inhabited the geological seas, and it is believed that the other echinoderms descended from them. A few now inhabit the deep seas. Some species are fixed by stems when young, and later break away and become free-swimming, others remain fixed throughout life.

The four classes of the branch echinoderms are Starfish (_asteroids_), Sea urchins (_echinoids_), Sea cucumbers (_holothurians_), and Sea lilies (_crinoids_).

[Illustration: FIG. 67.--CRINOID, arms closed.]

[Illustration: FIG. 68.--DISK OF CRINOID from above, showing mouth in center and vent near it, at right (arms removed).]

Comparative Review

Make a table like this as large as the page of the notebook will allow, and fill in without guessing.

===================+=========+========+=========+=========+========= | AMEBA | SPONGE | HYDRA | CORAL | STARFISH | | | | POLYP | -------------------+---------+--------+---------+---------+--------- | | | | | Is body round, two-| | | | | sided, or irregular| | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | What organs of | | | | | sense | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | Openings into body | | | | | | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | Hard or supporting | | | | | parts of body | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | How food is taken | | | | | | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | How move | | | | | | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | How breathe | | | | | | | | | | | | | | | ===================+=========+========+=========+=========+=========