CHAP. IV.] THE NERVOUS SYSTEM OF MOLLTJSKS. 81 



the oesophagus. The branchial ganglia are, moreover, 

 fused with them, instead of with the cerebral as they are 

 in Eolis. 



The nervous system in the Cephalopoda presents 

 many peculiarities, which can, however, be only very 

 briefly referred to here. Owing to 

 an extreme, amount of shortening 

 of their commissures, the principal 

 ganglia are closely aggregated in the 

 head. The nervous system is, in- 

 deed, more concentrated and com- 

 plex than in other Mollusks, and the 

 animals themselves are notable for 

 the high degree of development of 

 some of their sensory organs as 

 well- as for their great powers of 

 locomotion. 



The body of the Pearly Nautilus, 

 contained within the last chamber of 

 its coiled and loculated shell, is en- 

 veloped by a muscular mantle open 

 anteriorly, round the head and its 

 numerous sensory appendages. Ac- F , G . 27 . __^ ervous Sy8tem f 



Cording to Owen,* "the number Of the Common Slug (Solly after 



.,, , . , ., -~ , Baly.) A A, Cerebral ganglia ; 



tentacles With Which the Pearly B B, branchial ganglia and c, 



Nautilus is provided amounts to no 



less than ninety, of which thirty- 

 eight may be termed digital, four ophthalmic, and forty- 

 eight labial." The eyes, not so well developed as in 

 the Cuttle-fish, are also in relation with smaller optic 

 ganglia (fig. 28, o o). Near them are two hollow bodies, 

 regarded by Valenciennes as olfactory organs, the nerves 

 from which join the same ganglia. The situation and 



* " Lectures on Comp. Anat. and Physiol, of Invert.." p. 581. 



G 



