42 W. A. HEKDMAN. 



legists to be organs of respiration. They are not present in 

 all Nudibranclis, bnt in many cases they are very large and 

 conspicuous. They may be present along with true branchiae. 

 I find the cerata in the genera which I have examined to be of 

 two kinds : 



1. There are those which contain large diverticula of the 

 liver, as in the case of the genera Eolis and Doto. 



2. There are those which are essentially processes of the 

 body-wall, and have no connection with the liver, as in the 

 genera Tritonia, Ancula, and Dendronotus. 



The term '' cerata " may, as introduced by Lankester in his 

 article '* Mollusca,^^ ^ be employed for these processes in 

 general, while those in the first category might be specially 

 denoted as hepato-cerata, and those in the second as parieto- 

 cerata. All the forms which I have examined are either 

 distinctly hepato-cerata or are parieto-cerata. I have found 

 no intermediate conditions. In regard to their morphological 

 nature, if the fold of integument overhanging the foot in 

 Doris is to be regarded not as a mantle edge, but as an epi- 

 podial ridge (see Lankester, loc. cit., p. 655), then the smooth 

 or tuberculated dorso-lateral ridges in the genera Goniodoris, 

 Polycera, and Idalia, the larger row of lateral tubercles of 

 iEgires punctilucens, the lateral clavate processes of Triop a 

 claviger, the palisade-like cerata of Ancula, the branched 

 parieto-cerata of Tritonia and Dendronotus, and probably also 

 the hepato-cerata of Doto, Eolis, Proctonotus, and all other 

 forms, may be considered as epipodial papillse — outgrowths from 

 a more or less distinct epipodial ridge. 



The six common British genera, Doris, Ancula, Tritonia, 

 Dendronotus, Doto, and Eolis, show very different con- 

 ditions of the cerata and other dorsal processes, and form an 

 instructive series of types. The general anatomy of all these 

 forms is well known, thanks chiefly to the labours of Alder 

 and Hancock and of Rudolph Bergh, and many points in the 

 detailed structure of particular organs have been worked out 

 by Bergh, Vayssiere, Trinchese, and others; but the method 

 1 ' Ency. Brit.,' ninth edition, vol. xvi, p. 655. 



