744 ZOOLOGY SECT, 



Nervous system and sense organs. The ganglia of the 

 central nervous system are in all closely aggregated together round 

 the oesophagus, as already stated to be the case in Sepia ; and the 

 general disposition is the same as that described. In Octopus the 

 ganglia are much less sharply marked off. In Nautilus, as 

 already stated, there is less concentration, and distinct ganglia are- 

 absent. All the Dibranchiata possess highly developed eyes 

 similar to those of Sepia ; but in Nautilus the eyes are of a much 

 simpler character, each consisting of a sac opening on the exterior 

 by a small rounded aperture, lined internally by a two-layered 

 retina similar to that of Sepia, but without lens, vitreous humour, 

 or cornea. In the embryo of the Dibranchiata, the eye passes, 

 through a stage in which it is in the condition of an open cup 

 similar to the adult eye of Nautilus. Osphradia, are present, as 

 already mentioned, only in the Tetrabranchiata ; but in both the 

 Dibranchiata and the Tetrabranchiata certain sensory processes 

 or depressions conjectured to possess an olfactory function are- 

 developed on the head. Otocysts are universally present. 



All the Dibranchiata have two nephridia similar in character 

 to those of Sepia, and communicating with one another ; in Octopus, 

 they are completely united. In the Tetrabranchiata there are 

 four nephridia, each opening on the exterior. 



The sexes are distinct in all the Cephalopoda, and in addition 

 to the hectocotylised arm, there are frequently other external 

 differences between male and female. In all the Dibranchiata 

 the arrangement of the gonads and gonoducts is, as regards, 

 general features, similar to what we find in Sepia, In Octopus, 

 however, there are two oviducts instead of one, and in one other 

 member of the Octopoda (Elcdone moscliata) the same holds good 

 of the spermiducts. 



Development. The development of the Dibranchiata alone is. 

 known. The eggs are very large, containing a relatively large 

 amount of food-yolk. They are usually laid in masses or strings 

 embedded in soft gelatinous, or tougher, more leathery substance, 

 usually attached to some foreign body ; in some cases each egg. 

 enclosed in its gelatinous sheath, is attached by a longer or shorter 

 stalk. A cliorion or delicate transparent egg-membrane, in which 

 there is an aperture the micropyh immediately invests the 

 egg itself. In shape the egg is oval or spherical. The greater 

 part of the comparatively small quantity of protoplasm lies as 

 a disc-like elevation on the surface of the yolk on the side of 

 the egg at which the micropyle is situated. Continuous with this 

 germinal disc is a thin layer of protoplasm the peripheral proto- 

 plasm investing the entire ovum. 



Segmentation (Figs. C56 and 657) is incomplete, being confined 

 to the germinal disc. At an early stage in the process of division, 

 the blastoderm exhibits a distinct bi-lateral symmetry. At first. 



