56 W. A. HERDMAN. 



creeping over the surface of colonies of Alcyonium digi- 

 tatum. The specimens of Tritonia plebeia are marked 

 with many colours (none of them bright) including tints of 

 yellow, brown, blue, grey, black, and opaque white ; and when 

 examined in a vessel by themselves considerable differences 

 between individuals are noticed, but when in their natural con- 

 dition on the Alcyonium colony they are nearly all equally 

 inconspicuous. The colonies of Alcyonium differ consider- 

 ably 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 Tritonia plebeia do 

 not render it conspicuous, but are suited to the varying con- 

 ditions of the Alcyonium colonies. The small branched 

 cerata along the back of the Tritonia aid the protective re- 

 semblance 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 con- 

 spicuous and brightly coloured animal, but I find it at Hilbre 

 Island invariably creeping on the under surfaces of ledges and 

 stones on which are large colonies of the zoophyte Clava 

 multicornis, 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 

 sporo-sacs, that when the Doto remains still it is hidden to a 

 very remarkable extent.^ 



Dendronotus, again, with its large branched cerata and 



' Mr. Garstang tells me, in a letter just received (October), that at Ply- 

 mouth the specimens of Doto are not so highly coloured, and arc found upon 

 Calyptoblasts, Clava being rare there. 



