352 PHYSIOLOGY OF THE INVERTEBRATA. 



refracting homogeneous granules. These vesicles receive 

 nerves, and sometimes even rest on the central ganglia of the 

 nervous system. 



The eyes or ocular spots in the Asteridea are five in 

 number, and they are situated at the end of each ray. 

 These organs are spheroidal, pedunculated, and pigmented 

 prominences ; being expansions of the ectoderm, and con- 

 tinuous with the ambulacral or radial nerve (see Fig. 6i). 

 Each eye contains a number of clear oval bodies surrounded 

 by a pigment. These are said to represent the crystalline 

 cones of a compound eye. 



In the Ecliinidca the five ocular spots are situated on a 

 similar number of small plates, which form the apex of each 

 ambulacral segment. The ocular along with the five genital 

 plates surround the anus. 



The true function of the ocular spots in the Echinodcrmata 

 have been ascertained by Drs. Eomanes and Ewart.* The 

 Asteridea and Ecliinidca (but not the Ojjhiitridca) crawl 

 towards, and remain in, the light ; but when their ocular 

 spots are removed they no longer do so. On the other hand, 

 if only one of the five ocular spots were left intact, the 

 animal crawled towards the light as before. It may also be 

 stated that when their ocular spots are left intact, these 

 animals can distinguish light of very feeble intensity. 



The Tkichoscolices. 



In the Turhellaria the organs of touch are distributed over 

 the whole surface of the body, but the cilia are the chief 

 tactile organs. Some of these animals have auditory sacs 

 provided with otoliths ; and most of them possess eyes. 

 Many Planaricv have first of all in the embryonic state 

 pigmented spots in the place where, at a later period, eyes 

 with crystalline cones are developed. 



In the Botifcra, the tactile organs are cutaneous, and have 



* Philosophical Transactions, 1881, pt. 3, pp. 856, 873, 877. 



