Introduction to Animal Morphology. 267 



male lobe being above and in front, the female being 

 below and behind, but the ducts are united, as in Cy- 

 clas. In Pandora the two lobes, now separate, have 

 separate ducts, as in Clavagella. In the dioecious 

 Margaritana, islands of ovary have been found in the 

 testis. In Mytilus the gland is entirely overlapped 

 by the mantle lobes. In Anomia it is in the under 

 (right) lamella of the mantle. The gland is usually 

 under and beside, often inseparable from, the liver 

 (not so in Pinna), anterior to the hinder adductor, and 

 in the side of the visceral sac. The genital opening, 

 when separate from the organ of Bojanus, is on each 

 side at the base of the abdomen. Kellia, Montacuta, 

 and Galeomma are viviparous ; in others the eggs are 

 hatched in a gill pouch (Sphserium, Cyclas, &c.), or in 

 the interbranchial chamber. Most bivalves are proli- 

 fic ; one Unio lays 3,000,000, and one oyster 600,000 

 eggs in a single season. The eggs have usually an al- 

 bumen layer, thickened into a pillar-like process (mi- 

 cropyle) at one end ; development begins by a rotation 

 and cleavage of the yelk and the formation of a larva 

 with a unilobar velum, often with a flagellum. The 

 mantle (or in Cyclas the foot) is the first part to be 

 developed, and the other viscera follow. 



The 14,000 recent and fossil Lamellibranchs are divisible 

 into two sub-classes : — 



Sub-class I. Endocardines — extinct, with rough reticu- 

 lated shells, with unequal valves, the right attached to 

 foreign bodies ; umbones displaced from the margin to- 

 near the middle of the shell ; ligament internal ; hinge 



so that according to season an oyster appears wholly male or wholly female. 

 This statement has been called in question. 



