38 Dr. C. Collingwood on some points in the Economy 



But the papillae of the Eolididse are not branchiae either by 

 morphological structure or anatomical relation. In this family 

 the respiratory function is distributed with nearly perfect equa- 

 lity over the whole body, no one part being specialized for that 

 purpose. Here there is no duplication or lamination — no pro- 

 vision for multiplying the extent of surface — no distinct afferent 

 branchial vessel — no means to secure the imperfectly aerated 

 blood from contact with the venous stream contained in the 

 dermal sinuses — in fact, no one character which is essential to a 

 true gill. True, the blood is aerated sufficiently for the require- 

 ments of the animal ; but it is not effected by means of branchiae, 

 but by the general surface of the body ; and if the papillae assist 

 more than other parts, it is not because they are gills, but simply 

 because the skin is there more delicate, and allows of a more 

 perfect interchange between the blood there exposed and the 

 oxygenating influences of the surrounding medium. 



If to these important differences in the respiratory apparatus 

 it be added that the Eolididse in general have a much-divided 

 liver, possess no buccal glands, no cloak with its attendant spi- 

 cula, an uncomplicated systemic circulation, and an urticating 

 apparatus similar to that of the Actiniae (in all which respects 

 they contrast with the Dorididae and Tritoniadae), there appears 

 strong reason for believing not merely in the distinctness of the 

 family, but that they are more widely separated from the true 

 Nudibranchiata than has been generally admitted. At all events 

 they have no title to the term Nudibranchiata, with which order 

 they have apparently been associated chiefly on account of an 

 accidental similarity of form. 



With regard to the specific value of colour in the Nudibranchiata, 

 I may remark that, in the individual specimen which has been 

 the cause of these observations, the processes were of a bright 

 vermilion, while in the mature Triopa claviger they are described 

 as tipped with yellow or orange. Mr. Alder, however, does not 

 lay much stress upon this fact, but only remarks that "the 

 TriopcB we find in the north are more brightly coloured in the 

 processes than those from the south." Now the most brilliantly 

 coloured parts of the Nudibranchiata are usually the processes 

 and papillae, particularly the latter, in which there is a central 

 ramification of the hepatic cells ; and these it is which give the 

 character to the animal. This brilliant colour is associated with 

 fat, " always bearing a certain relation to the oily constituents 

 both of plants and animals" (Bennett). Various influences of 

 nutrition, seasonal conditions, and light, &c., may modify the 

 production of pigment, although this last agent is no doubt less 

 certain in its effects upon animals and plants living beneath the 

 water, and perhaps in comparative darkness, than upon those 



