ECHINODERMATA 217 



much significance. In the holothurians, on the contrary, the 

 body is capable of definite and considerable contractions, by 

 reason of both circular and longitudinal fibres. In forms with 

 incomplete skeletons, as the starfish, muscular fibres connect 

 the ossicles. There is a high degree of flexion of the arms of 

 crinoids and aphiuroids. There are also special muscles con- 

 trolling the water vascular system, the stomach, the mouth parts, 

 the spines and pedicellariae. The fibres are non-striate. 



252. The Nervous System consists of a ring around the 

 mouth and a radial nervous band in each arm supplying, by a 

 plexus of fibres and cells, all the radial organs. This system is 

 superficial (" ventral") to the radial water-tube (Fig. 99, r. n.) 

 and in the starfish preserves its connection with the ectoderm 

 from which it is in all forms derived. Other deeper lying, and 

 even aboral, nervous elements are described for some of the 

 members of the group. These elements, when present, have as 

 their function the innervation of the muscles of the interior and 

 of the aboral wall of the body. 



Sensory organs are not highly developed. The animals 

 show evidences of possessing a chemical sense (analogous to 

 taste and smell) by which the presence of food is detected. 

 This is apparently localized in the tentacles in such forms as 

 have them. A tactile sense is also present, and is most highly 

 developed in the tentacles, ambulacral feet, and other movable 

 outgrowths. At the tip of the antimeres of the asteroids, or of 

 the radial nerve (echinoid) are structures bearing pigmented 

 spots, which appear to be sensitive to light (eye-spots). These 

 cannot give more than a very general impression of light, by 

 means of the chemical changes induced in the pigment cells by 

 the action of the light. Starfish are more quiet during the day. 

 When the starfish is placed on its "back" it makes definite 

 coordinated movements to right itself. Experiments show that 

 the starfish is modified slowly by its experiences. That is to 

 say it learns to do certain things in certain ways. It retains 

 ("remembers") for at least a few days. 



253. Reproduction is wholly sexual. The sexes are distinct, 

 but the males and females are not often distinguishable by 



