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Vertical section through one arm of a starfish; b, ampulla; d, water canal opening at madreporic plate (st); i, radial water tube; m, mouth; ft, tube feet; py, digestive gland; stc, stomach. Davison, Zoology.
On the dorsal (aboral) side of the animal you will find a raised body about the diameter of a pencil. This body, which under the microscope has the structure of a very fine sieve, is called the madreporic plate. Through this plate water passes into a system of water tubes and reservoirs. These tubes extend ultimately into the rays, there ending in the individual tube feet.
METHOD OF LOCOMOTION. - If we could examine the connection of a tube foot with the system of water canals, we should find that water passing from the canals in the rays flows into a tiny receptacle connected with the tube foot. If the tube foot is to be extended, the muscles in the wall of the ampulla (the receptacle next the tube foot) contract. This forces the water out, closing a tiny valve on the side of the radial canal and sending the water into the tube foot, thus causing it to lengthen. The end of the tube foot is composed of a little disk of muscle, thicker at the outer side than on the inner. When this disk is placed against an object, and when water is withdrawn from the tube foot into the ampulla, the disk becomes fastened again by suction. If now the muscles in the wall of the tube foot contract, and this process takes place simultaneously and in hundreds of the feet, it can be seen that the body of the starfish is drawn forward a short distance. To release the tube foot water is pumped in from the ampulla by the process mentioned above. The same act repeated again and again results in locomotion at the rate of about six inches per minute in an adult starfish.
Diagram of nervous system of starfish; r, nervous ring
around mouth; n, radial nerves to each arm, ending
at the eye.
each ray, ending on the muscles of the tube feet. If this nerve ring is cut, then the different rays do not act together, but one set of tube feet pull in one direction and those of another ray pull in another, thus preventing locomotion. At the extreme end of each ray the nerve is found to end in a tiny eye spot. In your specimen it may show as a salmon-pink spot at the outer end of the ambulacral groove. This eye spot probably cannot distinguish form, but by means of it the starfish can distinguish between light and darkness.
ORGANS OF BREATHING. — The starfish spends most of its life in the water, although it may be found attached to rocks or under moist seaweed after the tide has receded. In common with all animals and plants it must have oxygen. Some oxygen is taken in with the water in the system of water tubes within the body, and some is taken out of the water by means of delicate fingerlike processes of the skin called branchia. The branchiæ protrude from between the spines of the dorsal surface. They may be withdrawn by the animal. They are much too delicate to withstand drying and cannot be seen on the specimen you are studying.
FOOD OF THE STARFISH. The food and method of feeding are of considerable interest to us because of the economic value of the clams, oysters, and other mollusks on which the starfish feed. Starfish are enormously destructive of young clams and oysters, as the following evidence, collected by Professor A. D.. Mead of Brown University, shows. A single starfish was confined in an aquarium with fifty-six young clams. The largest clam was about the length of one arm of the starfish, the smallest about ten millimeters in length. In six days every clam in the aquarium was devoured. The method of capturing and killing their prey shows that they, in some instances, appear to smother the mollusk by wrapping around it with their soft baggy stomach. The latest evidence on the subject, however, seems to show that they wrap around the valves of the mollusk and actually pull apart the valves by means of their tube feet, some of which are attached to one valve and some to the other of their victim. Once the soft part of the mollusk is exposed, the stomach envelops it and it is rapidly digested and changed to a fluid.' This it can do because of the five large digestive glands which occupy a large part of each ray, and which pour their digestive fluids into five pouchlike extensions of the stomach extending into each ray.
Damage to the amount of thousands of dollars is done annually to the oysters in Connecticut alone, by the ravages of starfish. During the summer months the oyster boats are to be found at work raking the beds for starfish, which are collected and thrown ashore by the thousands.
1 Observation by one of my pupils, F. T. Lacy, seems to show that in the case of the mussel the starfish inserts part of the stomach into the hole through which the byssus protrudes, and kills the mussel by means of the digestive fluid. When the shells gape, the starfish finishes the meal at its leisure.
REGENERATION. -It is no uncommon thing to find starfish with fewer arms than the normal number. In such specimens small arms are frequently seen, making it appear likely that, once having lost an arm, it might grow again. Such is indeed the case, the starfish having the ability to regenerate (grow anew) lost parts. If a starfish should lose all five rays, it is possible that under favorable conditions it might regenerate all its lost parts and become as active as ever in the destruction of shellfish. DEVELOPMENT. - Besides this asexual method of regeneration, the starfish reproduces sexually by means of eggs and sperms. The sexes are separate. The eggs are passed Starfish, showing regeneration of lost arms. Notice that in the lowest into the water; fertilization takes place specimen the arm is just begin- near the surface of the water, where the eggs ning to regenerate. and sperms are found in great numbers during the breeding season (June-July in Long Island Sound). Development proceeds at first as in the jellyfish, the egg segmenting to form a blastula and gastrula. Development does not proceed directly into a starfish, however, the animal being at first very unlike the adult. It swims freely until a limy skeleton is developed. The starfish larva eventually buds off a tiny star-shaped body which actually lives on the tissues of the larva, eventually becoming a tiny starfish no larger than a small pin head, At this stage of their existence they are found on eelgrass and other salt
water plants. A rapid increase in size takes place at this time; the young after two weeks at the most go to the bottom and begin their life there. It is estimated that if seaweed should be taken out of the water during the months of June and July in the region of Long Island Sound, enough young starfish would be killed to save over six million clams per week for each wagon-load of seaweed removed.
OTHER ECHINODERMS. - Other echinoderms, which are frequently seen, but which have less economic importance, are the sea urchins, sand dollars, sea cucumbers, and sea lilies or crinoids.
The sea urchin is found all along our northern coast, living in the tide pools along the shore and also in deep water off the coast.
The body of the sea urchin is almost hemispherical in general form, and is provided with a large number of long spines, which are movable. Locomotion is performed by means of the tube feet, as in the starfish, but the spines are used to some extent as levers. Food is ground up by means of a set of five strong teeth, placed just within the mouth.
The sand dollar is a very much flattened form of starfish, modified to withstand the pressure of the deep water in which it is found. Another deep-sea form is the basket star, a much-branched, five-armed form. Others are the brittle stars, so called from their habit of casting off their arms when disturbed.
The sea cucumber has a leathery skin, only fragments of lime being found scattered through it. The internal structure is much like that of the starfish. It has the curious habit of ejecting all its digestive organs when disturbed.
The crinoids were once far more common than they are to-day. Their fossil remains form a large part of some of our limestone rocks.
CLASSIFICATION OF ECHINODERMS
CLASS I. Crinoidea. Mostly extinct forms. Deep-sea attached forms. Example, crinoid (sea lily).
CLASS II. Asteroidea. Free moving, usually five-armed echinoderms. Example, starfish.
CLASS III. Ophiuroidea. Free moving echinoderms with movable arms distinct from disk. Example, brittle star.
CLASS IV. Echinoidea. Echinoderms with no free arms, spines usually movable
and well developed. Examples, sea urchin and sand dollar.
CLASS V. Holothurioidea. Soft bodied forms, the skeleton consisting of scattered limy spicules. Example, sea cucumber.
FOR THE TEACHER
Dodge, General Zoology, pages 87-95. American Book Company.
called the annelids.
The earthworm belongs to a group of animals They are so called because the body is made. up of a large number of rings or segments. (Lat. annulus, a ring.) Study a living worm in order to answer the following questions.
Careful study of several worms will show that the number of segments varies, the larger worms having more segments. Do the segments vary greatly in width? In shape?
Notice the color of the worm; is it like that of the ground in which it lives? Do the dorsal and ventral surfaces differ in color? Can you ac
The earthworm. Note the swollen area, the clitellum. Photographed by Overton.
count for this? The earthworm is an example of a bilaterally symmetrical animal, that is, one in which the right side is a counterpart of the left side. Compare with the starfish or sea anemone. Are the latter animals also bilaterally as well as radially symmetrical?
The two ends of the worm differ somewhat in appearance, the anterior end being pointed and the posterior rather flattened. Tests made with a pencil or other pointed object will show which end is more sensitive to touch. Which end usually goes first in crawling?
Measure an earthworm when it is extended and compare with the same worm contracted. Note the difference in length. This is accounted for when we understand the method of locomotion. Under the skin are two sets of muscles, an outer set which passes in a circular direction around the body, and an inner set which runs the length of the body. The body is lengthened by the contraction of the circular and the extension of the longitudinal muscles. How might the body be shortened?
Put your finger over the under surface of the worm. Notice the roughness. Examine the surface with a hand lens. Four rows of very minute
1 See Hunter and Valentine, Manual, page 131.