LAMELLIBRANCHIATA. 289 



with the inner wall of this gill. Inj^^.119. is shown a transverse sec- 

 tion of the right lobe of the mantle (a), and of the right pair of 

 gills in a fresh-water muscle (^nofZow), at the period when the external 

 gill is performing its marsupial function, and is laden with the ova : 

 b is the inner gill, c the outer gill, showing the basal canal and the 

 free margins of the partitions of the branchial cells in which the ova 

 are incubated. 



The developtaent of the ovum takes place in this temporary 

 marsupium : it has been studied by Carus in the Unto and by Quatre- 

 fages in the Anodon. Fig. 120. a shows the ovarian ovum of the 

 120 Unio Uttoralis before impregnation : the germinal vesicle 

 is seen at a, the coloured vitellus with the membrana 

 vitelli at i, the albumen at c, covered by the chorion. 

 Before the ovum reaches the branchial marsujiium, the 

 germinal vesicle has undergone the ordinary changes 

 which lead to its apparent destruction: the albumen in- 

 creases in quantity: pentagonal and hexagonal epithelial 

 cells are developed upon the surface of the vitellus, as 

 shown in Jig. b. ; vibratile cilia are developed upon them. 

 /^Movements in the albumen are then observed in the vi- 

 W^^^W cinity of these cells. They extend over the yolk ; the 

 ovunTand Currents in the albumen increase in strength, and, finally, 

 Anodon.' the yolk itself begins to revolve in the suiTOunding fluid. 

 This singular phenomenon was observed by Leeuwenhoek in 1695. 



Two parallel fissures next make their appearance, which, sinking 

 deeper into the yolk, divide that part, which is now included in 

 the rudimentary digestive sac, from the lobes of the mantle. 

 Calcification commences on the outer surface of these lobes, and 

 the first layer of the future shell forms a small triangular valve 

 on each side. When the rotation of the embryo is most active, 

 seven or eight revolutions may be observed in the minute. The 

 gills make their appearance as minute processes from the inner 

 surface of the mantle, near the angle between the pallial lobes and 

 the visceral mass. The development of the adductor muscle, single 

 at the beginning and near the hinge, is indicated by feeble actions of 

 opening and shutting the valves. The albumen during this develop- 

 ment is absorbed and assimilated ; and the embryo now distends the 

 chorion. In the young Anodon, long filamentary processes, twisted 

 together like a byssus, are developed from the visceral mass, which 

 mass seems to shrink in size as the lobes of the mantle and valves of 

 the shell increase. The thick, short, fleshy foot is subsequently de- 

 veloped in place of the byssiform filaments. 



The young Uniones and A?iodontes escape from the chorion before 



u 



