74 COMPARATIVE ANATOMY. 



perfectly defined ganglia, supplies branches to mantle, arras, 

 and viscera. 



Tunicata. A single, often yellowish ganglion, confined to 

 mantle, is placed between the oral and atrial openings. 



Lamellibranchiata. The ganglia are symmetrically ar- 

 ranged in three pairs. The members of the first, or labial 

 pair, are placed one on either side of the gullet, and are 

 united by a slight transverse band superiorly. It supplies 

 the parts about the mouth and anterior portions of the vis- 

 cera. The second, or branchial pair, is usually placed upon 

 anterior surface of posterior adductor muscle; it communi- 

 cates by long nerves with the labial pair, and gives branches 

 to the adjacent muscle, branchiae, and mantle. The two 

 ganglia are blended when the branchiae are united behind, 

 as in Unio (fresh-water mussel). The third, or pedal, is con- 

 fined to the foot. When this is absent, as in Ostrea (oyster), 

 the ganglia are wanting; but when it is largely developed, as 

 in Mytilus (sea-mussel), they often unite, forming a single bi- 

 lobed ganglion. 



Two diffuse oesophageal rings are thus formed, one occu- 

 pying the greater part of the visceral mass and defined in 

 front by the labial and branchial ganglia, the other by the 

 labial and pedal ganglia, and both on the sides by their com- 

 municating branches respectively. 



Gasteropoda. Ganglia with commissures forming oasoph- 

 ageal ring. The upper part may be recognized as the brain, 

 from which branches are distributed to the eyes, tentacles, 

 and adjacent parts of mantle. The sub-oesophageal ganglia 

 always connect with branchial and pedal ganglia by conspic- 

 uous commissures. A distinct set of ganglia intimately asso- 

 ciated with the mouth, gullet, and stomach, have been ob- 

 served in some naked-gilled forms, as Doris and Eolis. It is 

 probably analogous to a sympathetic system. 



Pteropoda. The nervous system resembles that of Gas- 

 teropoda, with the exception that the supra-oesophageal gan- 

 glia are absent. 



Cephalopoda. The ganglia are of large size, and are more 

 or less perfectly enclosed in a cartilaginous brain case. (See p. 



