of the Nudibranchiate Mollusca. 37 



of true branchipe, we cannot help perceiving- that these papillae 

 do not come under the same category with them ; for abranchia 

 or gill is an organ of an arborescent form, offering by its dupli- 

 cations and reduplications, and laminated structure, as much 

 surface as possible to the surrounding medium, and supplied 

 with an afferent and efferent system of vessels. But the papillae 

 of the Eolididse do not follow this morphological law ; for 

 although they are very numerous in certain species, great multi- 

 plication of them is not an essential element of the arrangement. 

 Thus in Eolis exigua the normal number is about ten, and in 

 JS. desjjecta only six or eight ; and from this they increase to an 

 indefinite number in E. papillosa, which stands at the other end 

 of the scale. Neither have these papillse any afferent branchial 

 veins in connexion with them ; so that neither morphologically 

 nor anatomically can they be regarded as branchia. 



Taking as illustrations the three British families of Nudi- 

 branchiata, viz. the Dorididse, Tritoniadse, and Eolididse, the fol- 

 lowing appears to me to be a fair statement of the value and 

 degree of specialization of their breathing apparatus : — The first 

 family (Dorididse) consists of two subfamilies, of which the Dori- 

 dinse possess true and perfect branchiae, external and naked, 

 situated upon the median line of the body, large and elaborate 

 in their construction, and easily performing the aeration of the 

 blood, although they may receive slight and unimportant assist- 

 ance from the general surface of the body, which is destitute of 

 appendages. The other subfamily (Polycerinse) possesses bran- 

 chiae of a smaller relative size, consisting usually of three or four 

 laminae only, which are inadequate of themselves to carry on the 

 respiratory process. But in these the body is not simple as in 

 the Doridinae, but diverges more or less into appendages, which 

 supplement the branchiae by increasing the aerating surface of 

 the body, and generally perhaps bear an inverse ratio in size 

 and number to the development of the branchiae. 



In the second family (Tritoniadae) the breathing organs, 

 although not equalling the branchiae of the Doridinse in the 

 perfection of their anatomical connexion, surpass them, and 

 more particularly those of the Polycerinse, in extent of laminated 

 surface ; they are numerous and effective, but placed along the 

 sides of the back, instead of being collected to one point of the 

 median line. Hence we find this family devoid of any secondary 

 processes which may serve as auxiliary aerating organs, such as 

 are found in the Polycerinse. Still the communication which 

 exists between the efferent branchial veins and the sinuses of the 

 skin which contain venous blood, affords a character which 

 places the Tritoniadse considerably below the Dorididse in the 

 scale of organization. 



