NUDIBRANCHIATA OF THE L.M.B.C. DISTRICT. 147 
Part IJ. Hprpop1AL NATURE OF THE CERATA. 
In a paper* laid by one of us before the British Associa- 
tion last September, it was suggested that all the various 
projections from the sides and back of Nudibranchs known 
as cerata are to be regarded as epipodial papille, or out- 
growths from a more or less distinct epipodial ridge. 
And Garstang + has independently arrived at the same 
conclusion in his recent Report upon the Nudibranchs of 
Plymouth Sound. 
Pelseneer has lately drawn attention { to the presence 
and condition of the epipodia in T’rochus and other Rhipi- 
doglossate Gastropods, but he does not consider these 
structures as being homologous with the large epipodial 
flaps of Aplysca and other Opisthobranchs and Pteropods. 
For these latter he uses the term parapodia, introduced 
by von Jhering, and open to the objection that it is already 
appropriated by a totally different structure in another 
eroup of animals. But the condition of the parts in 
Trochus is so very similar to that found in Polycera and 
Idalia, and the dorso-lateral processes of the two latter 
forms are so clearly comparable with the large lateral flaps 
of Aplysia, that we are inclined to regard all these projec- 
tions as being homologous structures, entitled to be con- 
sidered as epipodial in their nature. 
We now give figures of a series of transverse sections 
(Pl. VI.) for the purpose of showing the condition of the 
epipodial structures in a number of different forms of 
Nudibranchs. 
The typical epipodia are seen in Elysza (Pl. VI. fig. 1) 
* Herdman on the Struct. and Functions of the Cerata, &c., Brit. Assoc. 
Report, 1889 (abstract only), and published in full in Quart. Journ. Microsc. 
Science, vol. xxxi., p. 41. 
+ Journ. Mar. Biol. Assoc., vol. i. no. 2., p. 181. 
+ Sur l épipodium des Mollusques, Bull. Sci., 1888, p. 182. 
