TYPE MOLLUSCA. 353 



ences is found in the arrangement of the layers of the retina, the rods being 

 turned towards the light as is usual in Invertebrate eyes, while in the Verte- 

 brates they are reversed, the nerve-fibre layer lying above them, the light 

 of necessity penetrating it before reaching the rods. The structure of the 

 lens is again very different, being cellular and formed as an invagination of 

 the ectoderm in the Vertebrates, while in the Cephalopods it is a cuticular 

 structure. These are fundamental differences and may suffice to show 

 what is meant, but many other dissimilarities may readily be found. 



Otocysts also occur imbedded in a capsule forming part 

 of the cephalic cartilage. They have the characteristic Mol- 

 luscan form and receive a large nerve arising from the cere- 

 bral ganglion. Osphradia occur only in Nautilus, where they 

 form a pair of sensory papillae one of which lies at the base 

 of each of the more ventral ctenidia. Other Cephalopods, 

 though lacking these structures, are yet provided with special 

 olfactory organs in the form of a pair of fossae or grooves 

 lined by ciliated and sensory cells and situated above the eye 

 in the position occupied by the eye-tentacles of Nautilus 

 (see p. 358), from which they may possibly have been derived. 



The excretory organs consist of two comparatively large 

 sac-like nephridia except in Nautilus, in which, in harmony 

 with the number of ctenidia and auricles, there are four. In 

 Octopus and the other members of the group Octopoda the 

 two nephridia are quite separate from one another, but in the 

 group Decapoda, to which Loligo and Sepia belong, they are 

 placed in communication with one another by transverse 

 canals one of which may be produced dorsally into a large 

 sac occupying a great portion of the anterior region of the 

 body. The venae cavse and branchial veins lie between the 

 walls of this anterior sac and the paired posterior nephridia, 

 and along the course of the veins the walls of the excretory 

 sacs are richly folded (Fig. 154, ne), constituting the venous 

 appendages, for a long time considered to be the excretory 

 organs in their entirety. The posterior paired nephridia 

 present the same relations to the exterior and to the entero- 

 003! which exist in other Mollusca, opening by two distinct 

 apertures into the mantle-cavity on the one hand, and on the 

 other communicating with the large euteroccel which has 

 been shown to be the equivalent of the pericardial cavity of 

 the Gasteropods and Pelecypods. 



