520 



THE INVERTEBRATA 



The oesophagus is very narrow, but capable of distension as large 

 masses of food are passed through it into the muscular stomach with 

 its spiral caecum in which is a very elaborate ciliary sorting mechanism. 

 Here the food is mixed with the secretion of the digestive gland 

 ("liver"), a bilobed gland of solid appearance. The two ducts are 

 covered by tissue resembling the excretory epithelium of the kidney, 

 which was formerly termed the "pancreas ". It is now certain that this 

 tissue is excretory and that the main part of digestion is performed 

 by the juices of the digestive gland. The cephalopods differ from 

 the majority of invertebrates in that the semidigested food does not 

 penetrate into the cavity of the digestive glands but digestion is 

 completed in the stomach. The whole of the alimentary canal is 

 comparatively short and simple as might be expected from the 

 carnivorous diet of the animal. 



Fig. 382. Lateral view of the brain of a cephalopod (Eledone?) to show the 

 localization of function. After Buddenbrock. al.c. alimentary canal; buc. 

 buccal ganglion ; cer. the different divisions of the cerebral ganglion ; brae. 

 brachial ganglion; ped. pedal ganglion; vis. visceral ganglion; the various 

 reflex centres A for biting, B for swallowing, C for swimming forward, D 

 for creeping and climbing, E for closing and F for relaxing the suckers, G 

 for in-breathing and H for out-breathing. 



The nervous system of Sepia is of great interest from the large size 

 and intimate association of the ganglia round the oesophagus, which 

 form a genuine "brain" (Figs. 382, 383) in which special centres for 

 the co-ordination of vital activities and for the simple reflex actions 

 have alike been detected. In contrast to vertebrates there is a con- 

 centration of nerve cells in the brain, only a few outlying ganglia 

 being present. A "skull" has been developed for the protection of 

 this large nervous mass composed of a tissue very similar to cartilage, 

 which also forms the supports of the fins and tentacles. 



The brain consists of the following ganglia : dorsally the cerebral or 

 supraoesophageal, ventrally (i) tht pedal, divided into the brachial (the 

 motor centre for the tentacles) in front and the infundibular (supply- 

 ing the funnel) behind, and (2) the visceral supplying the mantle and 



