606 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



Fig. 340. 



ganglia. They appear to be peculiar to Patella, 

 but are found more distinctly developed in the 

 Cephalopoda. 



Fig. 340 is a repre- 

 sentation of the nervous 

 system of Cliiton Mur- 

 moratus from the dissec- 

 tion of Mr. Garner, in 

 which the annular form 

 " c of the nervous system is 

 very perfect. The animal 

 presents many points of 

 resemblance to Patella, 

 and there is essentially 

 the same arrangement of 

 its nervous system. The 

 absence of ganglia on the 

 upper part of the ring (i) 

 is attributed by Mr. Gar- 

 ner to the want of eyes 

 and of tentacles. 



Nervous system of Chiton In ^ty there is 

 Marmoratus. an anterior or cerebral 



B, pedal ganglion, ganglion resulting from 



C, branchial ganglion. the function of two 



D, pharynL'eal ganglion, above the oesophagus 

 i, upper portion of ner-( c fin 341) f rom which 



Zn. 1 )^ * febralgan - small I nerves pass to 

 form the pharyngeal gan- 

 glion (p) beneath the pharynx: from this two 

 nerves pass backwards to form the pedal gan- 

 glion, which also gives nerves to supply the 

 mantle (P), and in the posterior part of the 

 body there is an additional ganglion, the 

 branchial (B, Jig. 341). 



Fig. 341. 



Fig. 342. 



Nervous system of Aplysia. 



In Sci/lhea, according to Mr. Gamer, the 

 brain is entirely supra-oesophageal ; it appears 

 to be composed of four united ganglia, pro- 

 bably the cerebral and branchial. The foot has 

 become too insignificant to require appropriate 

 ganglia. Mr. Garner has noticed two minute 

 black spots, one on each side of the brain, 

 composed probably of black pigment, which 

 he considers to be rudimentary of eyes. 



Nervous system of Scylleea Pelagica. 



A, cerebral ganglion. D, pharyngeal ganglion. 

 c, d, i, visceral branches. 



In Limax ater (common slug) the nervous 

 system is apparently much more simple; but 

 on a little examination it will be found to con- 

 sist of the same essential parts. A large supra- 

 cesophageal ganglion, bilobed, constitutes the 

 brain (u,fg. 343), from each side of which a pair 

 of nerves passes downwards to join a large sub- 

 cesophageal ganglion, Fig. 343. 



which supplies nerves 

 to the respiratory sac 

 and to the foot or loco- 

 motive apparatus. And 

 the pharyn^eal ganglia 

 are, as in Patella, con- 

 nected with the anterior 

 ganglion. In the sub- 

 O3sophageal ganglion 

 we see, conjoined, the 

 pedal and branchial 

 ganglia. 



In Buccinum unda- 

 tum the principal ner- 

 vous mass is sub-ceso- 

 phageal, and from it 

 nerves pass to the bran- 

 chiae and viscera, and 

 also to the foot and 

 integument. The for- 

 mer nerves form a gan- 

 glion, which may be 

 regarded as the bran- Nervous system of the cum- 

 chial ganglion.]* mon black slu 9 (l"nax 



For the nervous sys- ^ . h , 

 terns of Pteropoda and gang lion. b, infra-ceso- 

 Cephalopoda, see the phageal ganglion, d, an- 

 articles under those ter ir sub - oesophageal 

 titles. ganglions. 



ARTICTJLA.TA. In taking a general survey 

 of the structure of the articulated animals, we 

 observe that their body is divided into a certain 

 definite number of segments, each one of which 



* See Garner's paper,, loc. cit. 



