WAIlXlXd fOLoKATIoX. 1063 



The family is tlierefore probalily iii()iu)pliy It-tit- (so far as anv 

 family can be), and it would seem that tlie power of colour 

 production is a single character, which, in a few of the cases in 

 which it has arisen, was associated with distastefulness or other 

 protective quality, and so, possessing a great survival value, led to 

 the foundation of the family. What colour or pa,ttern might be 

 produced selection cared not, so long as it were conspicuous in the 

 eyes of Tiudibranch enemies, and duly correlated with inedibility *. 



Hence the multitude of patterns antl colours fountl in this one 

 class, and the variation above mentioned. Possibly one colour 

 is much the same a.s another in the biochemistry of the animal, 

 though so different to the human eye t. 



The survival value of the devices is evident from the abundance 

 of species in tropical waters. In collections, while species are 

 numerous, individuals are usually i-ai-e, but the peculiar habits of 

 many nudibranchs, which suddenly appear in local swarms and 

 shortly disappear again completely, show that as in, e. g., the case 

 of the Sharks of St. Andi-ews Bay J, which were not known to 

 exist there until new fishing methods were introduced, the pro- 

 portions in which species occur in collections are not at all those 

 in which they live in the sea. Of the Chromodorids I remember 

 particulardy C. annulata, C sjjkesi, C, hilaris. and Ceratosoma 

 corniyeriom as occurring for a few days in quite local swarms, 

 then disappearing absolutely ; while during the past fortnight 

 a, hundred specimens of C. reiicidata [PL LXI. (p. 1U68)] could 

 quickly be collected within a few yai-ds of where I wiite, but 

 to-morrow there may be not one. 



The collection would be the more easy in that this species like 

 all (Jkromodorids^ crawls about in the open instead of skulking 

 under stones or in crannies aftei' the fashion of most of the class, 

 except of course at low tide when it would otherwise be killed by 

 the sun. 



In contrast with this, the general inconspicuousness of most 

 Opisthobranchs culminates in many cases in adaptations whicli 

 result in resemblance to surroundings so sticking arul so well in 

 accord with the observed habitat, that there can be no doubt as to 

 protection being afforded by them §. 



* Amitlst this diversity we find that in all Chromodorids theedg-^ of the mantle i.s 

 specially coloured. In others the frills and rhinophores are coloured differently from 

 the rest of the body, as in the case of C. diardii. 



t That the colour of marine animals is often connected with their essential physi. 

 ology or apparatus of reproduction is indicated by the many cases in which the ea;gs 

 of a species are deeply coloured with the principal body colours of the parents. 

 This is conspicuously the case with numerous species of Polychwta and Nudibranclis, 

 The Chromodorid hereafter described under the name of C rat.hculnta lays eg-gs the 

 colour of the yellow bands on the mantle. The purple band secretes mucus of tlie 

 same tint, so that probably the purple colour has its physiological use, as well as 

 warning colour value. 



X Mcintosh, W. C, " Notes from the Gatty Marine Laboratorv," Ann. Nat. Hist, 

 vol. X. p, 251, 1902. 



§ I have never had the fortune to possess a properly stocked aci'iarium in vv'iiich 

 (experiments up this .side of the theory conlil be undertaken. 



