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 DibrancJiiata there is a single, irregular, white 

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

 the auditory sac of the Tetrabrancliiata contains many 

 otoliths.* 



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

 degrees of development. They are absent in the fixed 

 MoUusca. Certain 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 Laindlihranchiata 

 have as eyes sometimes only 

 pigmented spots, and some- 

 times brilliant organs, dissemi- 

 nated on the edge of the 

 Fig. 72. — The Evii ok Pecten. ,,. 



paJlium. 



a — cornea, b — lens, c = sclerotica. '■ 



d = optic nerve (2). e = retina. I^ Pedeu, there are a large 

 /" = optic nerve (i). g = pigment number of simple emerald- 

 layer, k = vitreous humour. n j -j. i. i 



_ , green coloured eyes situated 



round the edge of the pallium. 

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

 retina, choroid, and vitreous humour. The eye is peduncu- 

 lated, and it has a double optic nerve. 



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

 various Zain cllibranchiata. 



The Scap]i(ypodct and Folyplacoplioni have no eyes. 



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

 simple eyes situated on the summits of the large tentacula. 

 The eye of Helix 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 



• Dr. J. D. Macdonald in Proc. Roy. Soc, 1855. 



