their Organs of Respiration, 241 



being formed by the ramifications of the branchial artery, 

 and the other by those of the vein. These minute ramifica- 

 tions can be ultimately traced to two great trunks which run 

 along the back of the branchial leaf, one (the arterial) 

 trends away to the auricle of its own side, to pour into it the 

 renovated blood, while the other is the large vein from which 

 the venous branchlets have departed.* 



The naked acephalous Mollusca (Tunicata Lam.) have two, 

 and only two, orifices in their outer tunic, which very often 

 open on the tips of two tubular projections, or papillae, placed 

 near one another. By one of these f , which is usually the 

 highest, and encircled within its rim with one or two rows of 

 slender tentacular filaments, or furnished with a valve, the 

 water necessary to respiration flows into a large visceral sac, 

 which, while it seems in part to perform the functions of a 

 crop, affords ample space for the display of the aerating 

 blood-vessels. The water, after being breathed, is in general 

 expelled at the same orifice by which it was sucked in ; and, 

 notwithstanding that the observations of several naturalists 

 seemed to prove the contrary, Cuvier was nevertheless in- 

 clined to conclude, from his anatomical investigations, that it 

 could not possibly be expelled from the other, which is the 

 anus. But the conclusion was erroneous ; for, since attention 

 was more particularly directed to the point, the water has 

 been seen to be propelled simultaneously from both apertures, 



* Manuel, p. 128. Such of our readers as are interested in the structure 

 of molluscous animals will not be displeased at the length of the following 

 extract from Cams : — " It is to be remarked, farther, of the large branchial 

 laminae of the freshwater muscle, that both pairs consist of an inter- 

 texture of vessels, arranged in a rectangular latticework, and covered by 

 a delicate membrane, whilst the two external are distinguished by a structure 

 which merits a particular description. Above each external lamina of the 

 gills is a duct proceeding from the posterior part of the foot towards the 

 anal tube, long ago described as an oviduct by Oken, and having on its 

 lower surface a long row of openings placed transversely, and forming the 

 entrances to the cells, or compartments, of the gills themselves. These 

 compartments are all arranged vertically in the gill, and separated from 

 each other by partitions : they appear as though they originated from the 

 mutual recession of the two membranous surfaces of the gill, which remain 

 connected only by the vertically disposed vessels that give rise to the septa : 

 they serve for the reception of the ova, which, coming from the ovary 

 placed within the foot, and not by any means formed in the gill itself, 

 are, however, lodged there j and there receive their farther developement, 

 as in a uterus. This is a remarkable instance of the connection between 

 the sexual and respiratory functions." (Comp. Anat. y vol. ii. p. 148, 149. 

 trans.) 



f In all the compound Mollusca, the branchial orifice of the component 

 individuals tends always to approach to the circumference of the system, 

 as the anus does as invariably to the centre. (Savigny.) 



Vol. VI. — No. 33. r 



