The Starfish
Fig. 52.—Starfish on a rocky shore.
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.
Fig. 53.—Plan of starfish; III, madreporite.
External Features.—Starfish are usually brown or yellow. Why? (See Fig. [52].) Has it a head or a 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 centre 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 centre 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.
Fig. 54.—Limy Plates in portion of a ray.
Fig. 55.—Starfish (showing Madreporite).
Fig. 56.—Water tube System of starfish.
m, madreporite; stc, stone canal; ap, ampulla.
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.
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.
Fig. 57.—Starfish, from below; tube feet extended.
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]).
Fig. 59.—Starfish eating a sea snail.
b, stomach everted.
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?
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 of a lighter colour, 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 nearly all sperm cells, has a vibratory, taillike 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 oysterman, 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.
Fig. 60.—Young starfish crawling upon their mother. (Challenger Reports.)
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.
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 to the 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.—Many starfish are brown or yellow. This makes them inconspicuous on the brown rocks or yellow sand. Brightly coloured species are usually chosen for aquaria.