250 



COLLEGE ZOOLOGY 



in the foot (Fig. 175, gon}. They are paired bunches of tubes 

 and open (g.ap) just in front of the renal aperture (r.ap) on 

 each side. The spermatozoa are carried out through the dorsal 

 siphon of the male and in through the ventral siphon of the 

 female. The eggs pass out of the genital aperture and come to 

 lie in various parts of the gills according to the species. The 

 spermatozoa enter the gill of the female with the water and 



fertilize the eggs. That por- 

 tion of the gill in which the 

 eggs develop is termed the 

 marsupium. 



The eggs undergo complete 

 but unequal segmentation. 

 Blastula and gastrula stages 

 are passed through, and then 

 a peculiar larva known as a 



FIG. 178. The glochidium stage , /.,. /T -,. 



in the development of Anadonla. glOC/lldlUm IS produced 

 ad, anterior adductor muscle ; by, bys- 

 sus ; 5, setae ; sh, shell. (From Lan- 

 kester, after Balfour.) 



178). The glochidium has a 

 shell (sfy consisting of two 

 valves which are hooked in 



some species; these may be closed by a muscle (ad) when a 

 proper stimulus is applied. A long, sticky thread called the 

 byssus (by) extends out from the center of the larva, and 

 bunches of seta (s) are also present. 



In Anodonta the eggs are fertilized usually in August, and the 

 glochidia which develop from them remain in the gills of the 

 mother all winter. In the following spring they are discharged, 

 and, if they chance to come in contact w r ith the external parts 

 of a fish, this contact stimulus causes them to seize hold of it by 

 closing the valves of their shell. The glochidium probably 

 chemically stimulates the skin of the fish to grow around it, 

 forming the well-known " worms" or "blackheads." While 

 thus embedded the glochidium receives nourishment from the fish 

 and undergoes a stage of development (metamorphosis) , during 

 which the foot ; muscles ; and other parts of the adult are formed. 



