32 NERVOUS SYSTEM. 



central gullet ; and the intestine, more or less bent upon itself 

 ends in a medial, vent nil amis. 



One or two pairs of salivary glands are, present in the dibran- 

 chiatcs, but wanting in Nautilus. The liver is always large ; and 

 the two hepatic ducts are generally glandular. A large and 

 sometimes spirally-wound Ciucum is frequently developed from 

 the commencement of the intestine, with which the hepatic ducts 

 communicate. The product of the salivary glands is uneolored, 

 limpid and acid, whilst that of the liver and pancreas is also un- 

 colored and acid, but rarely limpid. 



In the tetrabranchiata and the decapoda the (esophagus is 

 dilated into a crop separated from the stomach by a constriction. 

 The ciecum is small and rounded, and the intestine is twice bent 

 upon itself. The four-lobed loosely racemose liver is lodged in 

 the anterior portion of the perivisceral cavity, and is largely 

 developed. From either side of it spring biliary ducts which 

 open in the large blind-sack ; the pancreas is found at their 

 commencement. 



Nervous System. 



The Nervous System is mainly identical with that of the 

 gasteropoda. There are the three typical pairs, the cerebral, 

 pedal and visceral ganglia, surrounding the gullet and connected 

 by commissures ; whilst the nerves which supply the buccal 

 mass, the alimentary canal, the heart, the branchial and the 

 mantle develop additional local ganglia. 



The dibranchiates have the principal ganglia so closely con- 

 nected that the commissures are not readily perceived. The 

 optic nerves are well developed. The superior and inferior 

 buccal ganglia have each united in one mass, and the t\vo are 

 united by commissures around the (esophagus. The large nerves 

 of the arms, and those of the funnel or siphon, proceed from the 

 pedal ganglia, which are placed on the posterior side of the 

 gullet; with them are connected also the auditory nerves. From 

 the parieto-splanchnic or visceral ganglia proceed nerves along 

 the shell-muscles to the, anterior wall of the mantle, where they 

 enter the large <jnn</fi(i. xlfllat<i- ; these a re connected by commis- 

 sures, and send strong cords to the lius. Branches of the 

 parieto-splanchnic ganglia, following the vena-cava, supply the 



