524 



LECTURE XXI. 



Anodon. 



form of the branchial compartments. In 

 fig. 193. is shown a transverse section of 

 the right lobe of the mantle (a) and of 

 the right pair of gills in a fresh-water 

 muscle {A?iodon), 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. 



One feels surprised at first that the ova 

 should pass into the outer gills, when the 

 inner ones are nearer the two sexual ori- 

 fices. V. Baer pointed out the roundabout way in which the ova 

 glide along the basis of the inner gill as far as the cloaca, and from 

 thence through a particular canal of the mantle into the compartments 

 of the external gill : and the same acute observer has explained how, 

 in like manner, in Tellina, the semen ejected from the anal tube 

 enters by the ventral respiratory tube into the branchial chamber of 

 the female, and passes thence into the receptacles for the ova formed 

 by the gills. 



The external gills swell out to such a size in the fresh-water 

 mussel, Anodon, as to require a particular space in the shell 

 for their lodgment, and the valves of the female are consequently 

 more convex than those of the male. The same sexual character of 

 the shells is observable in the genus Unio, and the whole family 

 of the NaiadcB, as Kirtland and Lea have remarked in their beau- 

 tiful monographs on the North American species of these fresh- 

 water bivalves.* 



The actual penetration of the ovum by the spermatozoon, first ob- 

 served by the keen-eyed and accurate Martin Barry, who demon- 

 strated the capital fact to me in the year 18i3, has, amongst other 

 confirmations, received satisfactory ones from Keber's and Webb's 

 observations of the mode of impregnation of the ova of the river- 

 mussel {Anodon) ; and from those by Loven on Cardium pyg- 

 mceum.\ A tubular orifice, analogous to the micropyle in the 

 vegetable .ovum, is formed in or from the chorion: the sperma- 

 tozoa penetrate by this temporary orifice, which then closes, and is 

 indicated by a clear spot with a corrugated border. The sperma- 

 tozoon penetrates the yolk, and there, according to Barry, divides 

 into many parts.J 



CCCXXVI. t CCCXXIV. 



X CCCXXIV. a. 



