ECHINODERMATA 135 



found in the disk, and a radial nerve with branches, found in each 

 ray, in the integument covering the ambulacral groove. There are 

 also two radial nerve bands forming the deep nervous system, and a 

 third set of nerve elements, the aboral or coelomic nervous system, 

 extending along the roof of the arm superficial to the muscles. 



Sense Organs. — The tube feet and the spines have tactile nerves 

 associated with them and the whole animal is undoubtedly sensitive 

 to temperature. At the end of a ray one finds a red spot, the " eye,'^ 

 which is sensitive to light. Above it is a process, called the tentacle, 

 similar in appearance to the tube feet but without a terminal sucker. 

 The tentacles are olfacto-gustatory organs and more important to the 

 starfish than the so-called " eyes." 



Behavior. — The starfish senses food and is able to open oysters 

 readily. MacBride states that small bivalves are taken completely 

 into the stomach of the starfish, the empty shell being later expelled 

 through the mouth. 



MacBride quotes Schiemenz as follows: " A bivalve able to 

 resist a sudden pull of 4,000 grams will yield to a pull of 900 grams 

 long continued. A starfish can exert a pull of 1,350 grams- but must 

 raise itself into a hump in order to open an oyster." Turned upon 

 its back the starfish ordinarily uses certain rays to right itself. 



Economic Importance. Positive. — Starfish are used to a limited 

 extent as fertilizer, and have been extremely valuable in the study 

 of embryology, particularly in connection with the problems of 

 fertilization. Negative. The starfish injures oysters and other 

 molluscs by direct action, opening and devouring them or ingesting 

 small ones and expelling the shells from the mouth. One little 

 starfish ate over 50 young clams in 6 days (Mead). 



Class 2. Ophiuroidea. — The brittle stars or " serpent stars " re- 

 semble the true starfishes considerably, having a star-shaped body 

 with a central disc and five radiating arms. They have distinct 

 oral and aboral surfaces with the mouth in the center of the disc. 



The arms are slender and tapering, covered with plate-like 

 ossicles and lateral spines. The muscular system of the arms is 

 highly developed so that rapid movement is effected by their lateral 

 sweep. Pyloric ceca and anus are lacking, the madreporic plate is on 

 the ventral surface instead of the dorsal, the tube feet are tactile 

 instead of locomotor and the ampullae have disappeared. Serpent 

 stars secure their food by means of specialized, oral tube feet, two 



