3272 



ECHINODERMS 



the young Asterina, at all events, there is a prolongation of the fore part of the 

 body, which not only corresponds in position to the prolongation that becomes the 

 stem in crinoids, but actually serves for a short time as an organ of attachment. But 

 whereas in the crinoid the mouth moves upward to the surface opposed to this 

 organ of attachment, and there becomes surrounded by arms, which similarly face 

 upward, in the asteroid the mouth and its surrounding arms are bent downward 

 so as to face the sea floor, and the animal, instead of collecting its food from the 

 water above, extracts it from the mud below. Correlated with this mode of life, the 

 vent and madreporite are on the upper side of the body. So little is known of 

 the Paleozoic starfishes and their relations to later forms, that all classifications 

 must be regarded as provisional. Subdivisions have been based on the character of 

 the so-called pedicellariae. These are small pincer-like organs that occur in star- 

 fishes and sea urchins, on the surface of the test, as shown in the illustration be- 

 low. The movable spines covering the surface of these animals, and varying in size 

 from minute, delicate, bristle-like structures to long rods, which may be thin and 

 pointed, or thick, or even globular, are familiar to all. The pedicellariae are proba- 

 bly derived from the smaller spines; two of these 

 united at the base by a muscle, and slightly 

 curved so as to approach one another at the ends, 

 form the simplest kind of pedicellaria; and, by 

 gradual modifications of this type, all the varie- 

 ties may be derived. Many uses have been 

 ascribed to the pedicellariae, such as holding 

 pieces of food, or removing dirt from the surface 

 of the test. In some sea urchins they are pro- 

 vided with poison glands, which seems to show 

 that they serve as weapons of offense in those 

 cases. It has been considered that in sea urchins 

 their chief use is to catch hold of fronds of sea- 

 weed and keep them steady until the spines and 

 tube feet can be brought into action. The inner 

 surfaces of the forceps in the pedicellariae are re- 

 markably sensitive, and the blades close on any 

 minute object the instant their inner surfaces 



are touched by it. It seems, however, that in spite of the amount of attention de- 

 voted to these organs, we do not yet understand all their uses. Besides spines and 

 pedicellariae, starfish also have on the surface of the skin small tubular processes, 

 containing an extension of the body cavity. These have very thin, contractile 

 walls, and doubtless serve to assist respiration. All starfishes have tube feet, but 

 in some these have no suckers at the end, and in all cases those which are at the 

 ends of the rays are used only as feelers, and are stretched in the direction in which 

 the animal is moving. At the extremity of each arm is a single tube foot, which is 

 the first to be formed, and is known as the unpaired tentacle; this being always 

 stretched straight out. Immediately above this tentacle is a small eye, colored by 

 red pigment, and protected by small tentacles. 



a. Two-jawed, closed ; b. Two-jawed, open 

 c. Three-jawed. (20 times natural size.) 



