Notes oil South African Mollusca. 135 



rectum than does the ventricle. A single aorta arises from the hind 

 end of the ventricle and then divides into two vessels, one passing 

 backwards to the liver and adjacent organs, the other bending round 

 the anterior loop of the intestine and running forwards to supply the 

 cephalic region with oxygenated blood. 



Excretory system. — Kidney somewhat triangular, very broad in 

 the middle, but tapering in front and behind ; its walls much folded 

 internally. Anterior end scarcely projecting beyond the front of the 

 pericardium, but abruptly curved round towards the rectum, thus 

 forming a kind of hook. There is no ureter, but the kidney seems to 

 open on the posterior side of this hook. From this point a band of 

 modified epithelium extends along the edge of the mantle-cavity 

 as far as the respiratory opening, running back along the side of 

 the kidney, and then curving round the hind end of the cavity 

 and passing forward next to the rectum. It thus occupies the 

 position usually held by the ureter in the more highly organized 

 Sigmurethra. In Dorcasia a slight ledge or fold runs along the 

 side of the kidney and rectum, overhanging this band ; and in 

 Trigoneplirus, though this fold is not found on the side of the 

 kidney, it is sometimes present on the rectum, being especially 

 developed in T. lucanus (PL IV., fig. 4). 



Pedal gland opening below the mouth, and extending backwards 

 for two-thirds of the length of the foot ; usually embedded in the 

 pedal muscles, but emerging into the body-cavity for the greater 

 part of its length in some species of Dorcasia. Towards the hind 

 end, the gland is approximately circular in transverse section, with 

 a central duct. An irregular longitudinal fold projects from the 

 roof of the duct into the lumen (text-fig. 1, A, C). The wall of the 

 gland is composed of a very thick layer of radially disposed gland- 

 cells, within which is a thin layer of circular muscles immediately 

 surrounding the epithelium of the duct. This epithelium is 

 peculiar, consisting of small, very narrow cells, which project into 

 the duct like cilia. The fold, however, has a more ordinary columnar 

 epithelium, composed of broader cells with granular contents. The 

 centre of the fold is pigmented in Trigoncphrus porphyrostoma. 

 Further forwai'd the gland becomes flatter and much wider, the 

 internal fold being broadened out to form the roof of the widened 

 duct (see text-fig. 1, B, D). The gland cells do not converge to 

 open in a median groove in the floor of the duct with a longitudinal 

 ridge on each side of it, and the structure of the gland differs widely 

 from that of the European forms described by Andre.* 



* Eevue Suis-se de Zool., 1894, ii. pp. 291-348. Pis. XII, XIII. 



