NERVOUS SYSTEM. 75 



26.) The supra- and infra-oesophageal ganglia are connected 

 by commissures, as in Gasteropoda. In Nautilus, the latter 

 are two in number and placed one in front of the other. The 

 optic nerve arises from the superior ganglia; the olfactory 

 nerve, and branches for the tentacles and body, arise from the 

 inferior. In Argonauta, the ganglia are in a single mass in the 

 median line, the optic nerve arising from the lateral portions. 

 Brachial nerves enter the axis of the tentacles, and are fur- 

 nished with ganglia, which control the action of the suckers. 



ARTICULATA. The ganglia are arranged in the axial line 

 of body and joined by a double chain of commissures, ex- 

 cepting Eotifera and Entozoa. The < neural cord,' thus con- 

 stituted, lies upon the ventral aspect of the segments. Its 

 anterior portion commonly surrounds the oesophagus, form- 

 ing an cesophageal ring. As a rule, each segment retains a 

 pair of ganglia with its afferent and efferent nerves. 



Rotifera. The nervous system has been imperfectly made 

 out. It is probably not strictly conformable to that of the 

 articulate type. 



Entozoa. See p. 161. 



Annelida. The neural cord not fixed to ventral region, 

 but floats freely in viscera. Supra-cesophageal ganglion bir- 

 lobed; sub-cesophageal, conspicuous. Nemertes differs re- 

 markably from the other Annelida, in the nervous system 

 being without apparent enlargements, and composed of two 

 separate cords, one on each side of the body, which send 

 off lateral branches along their course. (Siebold.) 



Crustacea. The nervous cord is usually connected to ce- 

 rebral ganglia by an oesophageal ring. As a rule, the ganglia 

 are distinct; but in certain forms, where segments of the 

 skeleton are fused together, those portions of the cord cor- 

 responding to such segments form one mass, within which 

 are represented the elements of as many ganglia. In the 

 higher Crustacea, as in Homarus (lobster), the cord forms 

 two separate centres of ganglionic union, one for the head 

 and thorax, and one for the abdomen. 



Arachnida. The cord forms, as a rule, two enlargements, 

 one for the cephalo-thorax, and one for the abdomen. 



