16 A MONOGRAPH OF THE 



In connexion with the vascular system in the Borididce, there is a gland-like organ which 

 generally overlies the buccal mass. This is copiously supplied with blood by a branch from 

 the aorta. Its function is unknown, but it may probably be analogous to some of the vascular, 

 ductless glands of the higher animals. 



The flow of the blood is rapid; the pulsations of the heart varying, in the different 

 species, from fifty to a hundred in the minute. 



It is evident, from the state of the circulatory apparatus, that respiration is performed only 

 in part by the branchiae. In all the three families, the skin, which is covered with vibratile 

 cilia, acts as an imperfect accessory breathing organ, and thus the blood is returned to the 

 heart in a partially aerated condition. The branchiae are most highly developed in the 

 Doridida, in which they are always laminated; the blood is brought to them by a distinct 

 afferent vein, and, after circulating over the respiratory surface in a definite course, is returned 

 to the auricle by another distinct or efferent vein, there to be mixed up with that brought 

 from the skin. In the Tritoniadce the gills have a similar laminated structure, and the 

 branchial circulation is apparently as complete as in the Borididce ; but the efferent branchial 

 veins are not distinct, but communicate with the sinuses of the skin, and therefore transmit to 

 the auricle a mixed stream. The Uolididce have the respiratory organs less perfectly 

 organised; they are generally papillose, though sometimes branched. Here there are 

 apparently no afferent branchial veins ; the blood being at once drawn from the sinuses of the 

 skin into a vessel which passes up one side of the papilla : it then filters through fine cellular 

 tissue, under the aerating surface, into another vessel on the opposite side, and so passes on in 

 distinct vessels to the great efferent, branchio-cardiac trunks, which conduct it, mingled with 

 the blood received from the sinuses of the skin, to the auricle. The gills, whether laminated 

 or papillose, are clothed with vibratile cilia. 



Renal Organ. An excreting organ, which, from analogy there can be no doubt is of a 

 renal character, exists in all the Nudibranchs, though little is known of its anatomy, except in 

 the Doridida; in all, however, the portal heart and the excretory orifice have been 

 detected. This orifice is invariably associated with the anus. The renal organ is, in the 

 Borididce, a large membranous sac, more or less branched, lying immediately below the 

 pericardium, and having its floor firmly attached to the liver-mass. It is supplied with 

 arterial and venous blood, by the systemic and portal hearts ; both of which send numerous 

 vessels to ramify in its walls. 



Nervous System. This presents a high degree of concentration, — perhaps higher than in any 

 other group of Mollusks, — and is divided into two very distinct portions-; — one, the cephalic 

 or excito-motor ; the second, the splanchnic or sympathetic : these two portions intercom- 

 municate at several points. The cephalic ganglia are situated at the origin of the oesophagus, 

 and naturally divide themselves into two sets, which may be denominated respectively 

 oesophageal and buccal, or supra- and infra-oesophageal. The latter are attached to the 

 buccal mass ; the former are more immediately related to the oesophagus, upon which they 

 are always seated, and about which they, with their commissures, form one or more constricted 

 collars. The two sets of ganglia are also interconnected. 



The oesophageal set is composed essentially of three pairs of principal ganglia, howsoever 

 they may be fused or blended together, and of one or two accessory pairs ; and, in the 

 Borididce, there is an additional visceral ganglion. The principal ganglia are the cerebroid, 



