Didelphia, the alternative name of the group was given in allusion to the circumstance that the uterus has two separate openings; while other features are the inclusion of the openings of the alimentary canal and the urino-genital sinus in a common sphincter muscle, and the position of the scrotum in advance of the penis.
P. Hill, "Contributions to the Morphology of the Female Urino-genital Organs in Marsupialia," Proc. Linn.
Each nephridium in the oyster is a pyriform sac, which communicates by a narrow canal with the urino-genital groove placed to the front of the great adductor muscle; by a second narrow canal it communicates with the pericardium.
The genital duct opens by a pore into the urino-genital groove of the oyster (the same arrangement being repeated on each side of the body) close to but distinct from the aperture of the nephridial canal.
Hence, except for the formation of a urino-genital groove, the apertures are placed as they are in Anodonta.
Enamel-folds; the stomach has a large glandular mass situated to, the right of the oesophageal orifice; the anal and urino-genital.
The urino-genital tubes arise from the posterior angles of the pericardium, pass first forwards, then backwards, and unite to open by a common opening into the cloaca below the anus except in A 8 C D FIG.