CEPHALOPODA. 1043 



The coelom is very extensive, and comprises the gonoccele and 

 the pericardial ccelom ; these cavities communicate freely with one 

 another, and are only separated by an incomplete septum, which is 

 atrophied in Sepia. 



In the Dibranchia there are 2 renal capsules, ventral and super- 

 ficial ; these two kidneys are attached to one another in the median 

 hue in the Octopoda, and they communicate to a greater or less extent 

 with one another in the Decapoda, with the exception of Spirula. 



In all the Cephalopoda the essential parts of the nervous system 

 are centralized in the head, round the initial part of the oesophagus ; 

 in the Dibranchia the nerve-centres are almost completely enclosed 

 in a cartilaginous capsule, and consequently many nerves transverse 

 the cephalic cartilage at their origin. 



The eyes are situated on the sides of the head, and are generally 

 sessile. They are. however, pedunculated in many embryos and in 

 some adult forms. In the Dibranchia the cavity of the eye is closed, 

 as it is in the majority of Gastropoda, and the ocular globe consists of 

 the same essential parts viz., retina, cornea, and crystalline lens 

 with various accessory parts added, making it a very complex and 

 perfect organ of vision. 



In all the Cephalopoda the sexes are separate, and sometimes there 

 is a well-marked sexual dimorphism. As a rule, the males are more 

 slender or smaller than the females. The maximum of sexual di- 

 morphism is found in Argonauta, in which genus the males are much 

 smaller than the females ; the latter may attain to fifteen times the 

 length of the other sex, and they have an external shell and the cha- 

 racteristic enlargement of the dorsal arms, both of which features are 

 absent in the males. Generally speaking, the males are also dis- 

 tinguished by the phenomenon of hectocotylization, which consists 

 in a curious modification for copulatory purposes of a part of the pedal 

 circumoral crown. 



The eggs are laid shortly after copulation. In the Dibranchia the 

 eggs are aggregated together, but in the Octopoda and in Sepia, Sepiola, 

 and Rossia each egg has a separate envelope, whereas they are united 

 to form longer or shorter gelatinous strings. In Eledone only about 

 60 eggs are laid at one time, in Polypus more than 100, and some species 

 of Loligo lay more than 40,000 eggs. Some Octopods are incubatory : 

 the female Argonauta, for example, protects the eggs in the shell 

 peculiar to her sex. 



Of the cuttlefishes, some, like Argonauta and Nautilus, are cha- 

 racteristic of warm seas, whilst the majority are cosmopolitan, their 

 enormous numbers in the great oceans being only occasionally in 

 evidence by their destruction of fishes on the lines, by the occurrence 

 of their beaks in the stomachs of numerous fishes (from the cod to 

 sharks), and by their forming the chief article of diet of the sperm- 

 whales. 



The Cephalopoda date back to the Cambrian epoch. 



