632 REPORT — 1889. 



small pieces of the one from those of the other. Then in Tritonia and Dendronotus 

 the cerata agree in structure ■with those of Ancula, and not with the true hranchise 

 of Doris and Ancula. 



Dendronotus is the form in which it might be most readUy believed that the 

 cerata have acquired a branchial function, but a close comparison of sections shows 

 that these processes are not more vascular than the general body wall, and have 

 not even so many lacunae as some parts of the dorsal and lateral integument. 

 Hence, although they may by their extended surface aid somewhat in respiration^ 

 still they cannot be regarded as in any way specialised branchiae. 



Then in Eolis and in Doto, although from their relatively very large size the 

 cerata may be of importance in respiration, it is merely as being an extension of 

 the general integument, and not as being special respiratory organs. In these two 

 genera also they have been made use of to accommodate the greater part of the 

 liver, and have probably become considerably enlarged. 



But I believe that in addition to these minor functions the cerata of the Nudi- 

 bvanchiata are of primary importance in giving to the animal, by their varied 

 shapes and colours, appearances which are in some cases protective and in others 

 conspicuous and attractive, and in this it seems to me we have an explanation of 

 the extraordinary development of these otherwise mysterious processes of the 

 dorsal body wall. 



To take a few cases : Tritonia (or Candiella) plebeia is fairly abundant at 

 Puffin Island and at Hilbre Island, near Liverpool, and is always found in those 

 localities creeping over the surface of colonies of Alcyo7iium digitatum. The 

 specimens of Tritonia are marked with many colours, including tints of yellow, 

 brown, blue, grey, black, and opaque white; and when examined in a vessel by 

 themselves, considerable difl'erences between individuals are noticed, but when in 

 their natural condition on the Alcyonium colony they are nearly all equally incon- 

 spicuous. The colonies of Alcyonium differ considerably amongst themselves in 

 tint, some being whiter, others greyer, and others yellower than the rest ; different 

 parts of the same colony also vary in appearance on account of the different states 

 of expansion of the polypes, and on account of irregularities of the surface, and 

 of adhering sand and mud ; so that the varieties of colouring found in the Tritonia 

 are suited to the various conditions of the Alcyonium colonies. The small-branched 

 cerata along the back of the Tritonia aid the protective resemblance not only by 

 contributing to the general colouring but also by their similarity in appearance 

 to the crown of tentacles of the partially expanded polypes. They are placed at 

 just about the right distance apart, and have the necessary tufted appearance. 



Then, again, Doto coronata, when isolated, is a very conspicuous and brightly- 

 coloured animal, but I find it at Hilbre Island invariably creeping on the under- 

 surfaces of stones on which are large colonies of the zoophyte Clava multicm'nis, 

 and in that position the Doto is not readily seen. The gay appearance of this 

 nudibranch is mainly due to the large and brightly-coloured cerata, and these 

 agree so closely in their general effect with the upper ends of the zooids of Clava, 

 covered with the numerous tentacles and the clusters of sporosacs, that when the 

 Doto remains stJU it is hidden to a very remarkable extent. 



Dendronotus again, with its lai-ge branched cerata and its rich purple-brown 

 and yellow markings, is a handsome and most conspicuous object, but I have fre- 

 quently found it amongst masses of brown and yellow zoophytes and on purplish- 

 red seaweeds, where it was very completely protected from observation, and I did 

 not for several seconds recognise what I was looking at. Now these are all cases 

 where the colouring is protective, and I have no doubt there are many other 

 similar instances to be found amongst the Nudibranchiata, but the species of Eolis 

 appear to be in a different category. They are noted for the very brilliant hues of 

 their dorsal papillae, and they are always conspicuous so far as I have noticed, 

 even under their natural conditions, Then, again, the species of Eolis are rarely found 

 hidmg in or on other animals, they are not shy and they are active in their habits 

 — altogether they seem rather to court observation than to shun it. When we 

 remember that the species of Eolis are protected by the numerous stinging cells in 

 the cnidophorous sacs placed on the tips of all the dorsal papillae, and that they 



