LAMELLIBRANCIIIATA. 525 



In all the bivalves hitherto observed the yolk is a germ-yolk only, 

 and is divided concurrently with each spontaneous fission of the 

 contained impregnated germ-cell. Two superficial cells, alone, seem 

 not to participate in this process, but, after the formation of the germ- 

 mass, are changed into triangular discs, the rudiments of shells : the 

 rest of the surface developes a ciliated epithelium, by which the 

 embryo rotates in the ovum. In Myt'didce a swimming disc is next 

 developed, fringed with long cilia, and an exploratory filament is deve- 

 loped in association with the increased locomotive power. 



In Anodon the embryonic mass soon divides itself, but not quite com- 

 pletely, into two halves*, each half being supported by its own valve. 

 It looks like an attempt at spontaneous fission, — some lingering mani- 

 festations of parthenogenetic force. In fact, each of these embry- 

 onal moieties has its own mouth, which is situated near the hinge, 

 beset with cilia, and its own intestine : on the inner sides of the cleft, 

 at the angle which the two embryonal moieties form at the future 

 hinge of the shell, there rises, on each side of the adductor-muscle, 

 a hollow byssiphorous organ, from which two transparent byssus- 

 filaments pi'oceed. The inner surface of each moiety is also beset 

 with three tentacle-like stifl' points, with their base surrounded by a 

 thick border. Near the hinge there passes the broad adductor- 

 muscle above mentioned from one embryonal half to the other, wliich, 

 after the young bivalve has quitted the egg, manifests its contractile 

 force by closing the valves ; these have now become slightly concave, 

 but continue triangular in shape. Each embryonal moiety has, like- 

 wise, at first, its own distinct though simple heart ; and it is by the 

 approximation and ultimate fusion of the two ventricles that the 

 common rectum of the originally distinct intestines is intercepted. 

 The rest of the canal blends with its fellow as the visceral mass grows 

 up from the bottom of the cleft, chiefly by the progressive increase 

 of the testis or ovarium \ ; and thus two sexless individuals combine 

 to form one with sexual organs. 



M. de BlainvilleJ denied the passage of the ova of the Lamel- 

 libranchs into the gills, and maintained their direct exclusion from 

 the shell ; on the faith of which assertion Jacobson § was led to 

 suppose the odd-shaped embryos in the bi'anchial marsupium to 

 be parasites, as the elder Rathke || had long previously described 

 them to be. But, whilst many true and singular parasites have 

 been detected and described in the Laraellibranchs, the real na- 

 ture of the Glochidium of Rathke has been well traced out : the 



• CCC^XXXIII. t CCCXXXIV. p. 312. 



X CCCXXVII. p. 137. § CCCXXIX. p. 22. 



II CCCXXVIII. p. 166. t. X. fig. 3. 



