368 



PHYSIOLOGY OF THE INVERTEBRATA: 



not situated in the cranial cartilage. In both orders, the 

 auditory nerve gives rise to nerve-filaments within the sac ; 

 and in the BibrancMata there is a single, irregular, white 

 otolith of a crystalline texture (CaOOj). On the other hand 

 the auditory sac of the Tetrahranchiata contains many 

 otoliths.* 



In the Mollusca, organs of sight are met with in various 

 degrees of development. They are absent in the fixed 

 Mollusca. Certaia of these, which in the state of mobile 



larvae had eyes, lose them by 

 degeneration when they have . 

 become immobile. Certain spe- 

 cies of the LamellihraTwhiata 

 have as eyes sometimes only 

 pigmented spots, and some- 

 times brilliant organs, dissemi- 

 nated on the edge of the 

 pallium. 



In Pecten, there are a large 

 number of simple emerald- 

 green coloured eyes situated 

 round the edge of the pallium. 

 Each eye (Mg. 72) consists of a cornea, lens, sclerotica, 

 retina, choroid, and vitreous humour, 

 lated, and it has a double optic nerve. 



The table on p. 369 gives the colour, &c., 

 various Zamellibranchiata. 



The Scaj)hqpoda and Folyplacophora have no eyes. 

 In the Pulmogasteropoda {e.g., Helix) there are a pair of 

 simple eyes situated on the summits of the large tentacula. 

 The eye of ITelix consists of the following parts : sclerotica; 

 choroid, lens, cornea, vitreous humour, and an optic nerve 

 which expands into an outer and inner retina. The eye in 

 these animals is much more highly developed than the simple 



Fig. 72.^The Eye of Pecten. 

 '. = cornea, i = lens, c = sclerotica. 

 d = optic nerve (2). e — retina, 



/ = optic nerve (i). ff = pigment 

 layer. k = vitreous humour. 



X' = eye-stalk. 



The eye is peduncu- 

 of the eyes of 



* Dr. J. D. Maodonald in Proc. Hoy. Soc, 1855. 



