ECHINODERMATA. 211 



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

 superficial (" ventral ") to the radial water-tube (Fig. 97, r. n.) 

 and in the star-fish 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, 



247. Reproduction is wholly sexual. The sexes are dis- 

 tinct, but the males and females are not often distinguishable 

 by external characters. The sexual organs, ovaries or testes, 

 are lobed bodies occurring usually in pairs in an interradial 

 position. These open by pores also interradial, and usually 

 dorsal (Fig. 97, r. p.). There are typically five pairs of genital 

 glands, but in the holothurians the number is reduced to one. 

 Fertilization takes place outside the body, and usually the 

 development is wholly independent of the parent. In some 

 instances however the parent has special pouches in which 

 development proceeds. 



248. Development. The fertilized ovum undergoes total 

 and practically equal segmentation, producing a ciliated 

 blastula. Gastrulation occurs by invagination resulting in 

 ectoderm and entoderm. The mesoderm is formed in two 



