298 



CEPHALOPODA. 



( )\\ sjannikow (and also probably the auditory plate which only 

 develops later). In examining the formation of this terminal sensory 

 apparatus, Grenacher thought he could also recognise the nerves 

 which give off branches to the cells in the form of delicate fibrous 

 strands. 



While the internal structure of the otocysts is thus developing, 

 the organs change their position, gradually shifting from a lateral 

 position to below the funnel (Fig. 141 A-C) where, as large closed 

 sacs, they are found in close contact with the pleuro- visceral ganglia 

 (Figs. 133, ot, p. 283 and 143, ac). Finally they come into close 

 contact with one another and flatten by mutual pressure, as was 

 observed in Grenacher's embryo and in Sepia. Their definitive 

 position being attained, the cephalic cartilage develops round them. 



The Eyes. 



The origin of the eyes in the Cephalopoda has been carefully 

 studied by Grenacher (No. 14), Ray Lankester (No. 2 ( .'). and 



BOBRETZKY (No. 4). 



Figs. 115-119, pp. 253-261 will help the reader to understand the 



orientation of the eye-rudiments in the embryo. These organs 



i >riginate in connection 

 with the large swellings 

 (Fig. 115, mi) as two 

 large, rather shallow 

 ectodermal depressions 

 (Fig. 142). The floor of 

 each depression soon 

 thickens considerably and 

 its margins grow up and 

 over it towards the centre 

 (Fig. 142 B). A vesicle 

 is thus produced with 

 a thin outer and thickened 



inner wall and this is connected with the exterior by a small aperture. 



The inner wall of this vesicle yields the retina, while the outer wall 



yields a part of the lens and the ciliaiy body. 



It is an interesting fact that this stage of development is retained 



throughout life in the eye of one Cephalopod, Nautilus (Fig. 145). 



The adult eye, in Nautilus, corresponds to the primitive optic vesicle, 



the cavity of which is lined by the retina, i.e., modified ectoderm, and 



-ri«l 





Fig. 142. — Transversa sections through two stages 

 of the eye in Loligo (after Ray Lan'kksteu, from 

 Balfouk's Text-book). 'Pin- ectoderm is repre- 

 sented dark. 



