TYPE MOLLUSC A. 315 



modifications of portions of the foot, being innervated from 

 the pedal ganglia. In these forms also fringed or simple pro- 

 cesses of the posterior portion of the body occur which serve 

 as respiratory organs, though they are not homologous with 

 the true branchia which in Pneumoderma coexist with them. 



Suborder Nudibranchia, 



In the Nudibranchs the visceral hump has undergone 

 elongation parallel with the long axis of the foot, from which it 

 is not distinctly marked off, and an apparent bilateral sym- 

 metry is manifested by the body. This condition, however, 

 is evidently entirely secondary, as is shown by the structure 

 of the heart and nephridium, in which the usual asymmetry is 

 well marked. There is no shell, mantle, or ctenidia. Adaptive 

 branchiae are, however, frequently developed, as in Pleurophyl- 

 lidia, where they form a series of folds 

 which lie in a groove at the side of the 

 body and recall somewhat the arrange- 

 ment in the Chitonidse, or in Doris, where 

 they surround the anus, which has a 

 dorsal position, and form a circle of pin- 

 nate processes. In the pelagic Phyllirhoe 

 and in the creeping Limapontia, however, 

 there is no trace of respiratory organs. 

 Many forms (Fig. 141), such as ^Jolis, 

 Facellina, and their allies, bear upon the 

 dorsal surface of the body numbers of 

 finger-like processes usually arranged in 

 bunches, and frequently brightly colored. 

 These cerata frequently enclose branches 

 from the intestine which correspond to 

 the digestive gland of other forms, and pia 141 _ N UDIRRAN . 

 bear at their extremities a sac in which CHIATE OPISTHO- 

 are developed nematocysts. These organs BRANCH (^Eolid). 

 are usually richly provided with blood- 

 vessels, and are probably respiratory in function, though 

 the presence of nematocysts renders it probable that they 

 are also protective an idea which is confirmed by their 



