5^ LECTTJEE XXT. 



young Anodons, so denominated, quit their marsupium' usually late 

 in Septembei', after an intrabranchial development of two ^g^ 

 or three months. The young animal, when excluded, 

 uses the prehensile or adhesive filament, to anchor itself 

 to the shell of the parent or to some foreign body. 



Pi'of. Carus* has ably traced out the course of deve- 

 lopment of Unio litoralis. Fig. 194, A, shows the unimpreg- 

 nated ovarian ovum. Before the ovum has reached the 

 branchial marsupium, the germ-mass has been formed, 

 and some pentagonal and hexagonal cells are developed at 

 its periphery, supporting vibratile cilia. The quantity of 

 albumen has increased. At first movements may be ob- 

 served in that part of the albumen, which is contiguous to 

 those superficial ciliated cells. As these extend over the 

 germ-mass the currents of albumen increase in strength, 

 and, finally, the yolk itself begins to revolve in the surrounding fluid 

 in the direction marked by the arrow in b. This singular pheno- 

 menon was first observed by Leuwenhoek in 1695. The embryos 

 have now penetriited the gilh Two parallel fissures next make their 

 appearance, which, sinking deeper into the germ-mass, divide that 

 part into two, and mark out at the bottom the rudiment of the vis- 

 ceral mass, which is subsequently to rise between the lobes of the 

 mantle. Calcification commences on the outer surface of these lobe?, 

 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 ciliated wavy folds from the inner surftice 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 attempts at opening and closing the 

 valves. Tlie albumen during this development is absorbed and as- 

 similated, and the embryo now distends the chorion. In the young 

 Anodon {Jig. 194', c), long filamentary pi'ocesses twisted together like 

 a byssus, are developed from the visceral mass : tlie thick short fieshy 

 foot is subsequently developed in place of the byssiforra filaments. 

 Both the young Uniones and Anodo?ites escape from the chorion 

 before they quit the marsupium, and may be observed swimming 

 freely about in the cavity of the external gill. They are so different 

 in form from their parent, and so singularly located, as readily to 

 suggest the venial error of Rathkd respecting their nature and rela- 

 tions. The hooked apex and the spines, with which the shell of the 



* CCCXXX. p. 43, tab. 



