596 THE INVERTEBRATA 



and does not enter the liver. In this respect the cephalopods appear 

 to differ from the majority of invertebrates. 



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. 407, 408) 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. For the protection of this large nervous mass a "skull " 

 has been developed composed of a tissue very similar to cartilage, 

 which also forms the supports of the fins and tentacles. The nerve net 

 found in the foot of gasteropods is absent. 



cer. 



VIS. 



Fig. 407. 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 brain consists of the following ganglia : dorsally the cerebral or 

 supraoesophageal, ventrally (i) tho^ 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 

 the visceral hump. The cerebral ganglia are much more differentiated 

 than any of the others. They can be divided into separate regions 

 which co-ordinate the movements of organs for the performance of 

 such complicated actions as feeding, swimming and creeping. In the 

 visceral ganglia there are also two sharply defined centres which 

 control the movements of the whole mantle in in-breathing and out- 

 breathing respectively as well as numerous small centres, the stimu- 

 lation of which causes contraction of small muscle patches in the 

 mantle, while in the brachial ganglia there are separate centres for 

 gripping by the suckers and for letting go. 



