454 



PHYLUM MOLLUSCA 



accumulate in crowds in the outer gill-plates. The repro- 

 ductive organs are branched and large ; there are no 

 accessory structures ; the genital aperture lies on each side 

 under that of the ureter. 



The ova pass from the ovaries in the foot, and appear to 

 be moved to the exhalant region, whence, however, they do 

 not escape, but are crowded backward till they pass into 



Fig. 257. — Glochidium larva of the fresh-water mussel. 



SH., Valve of the shell; MU., adductor muscle-fibres workmg 

 the valves ; T., a tooth on the margin of the sheU ; B., a 

 thread of glutinous byssus, which effects fixation ; C, a 

 tuft of sensitive cells inside the mantle. The size of the larva 

 is that of a small pin's head. 



the cavity of the outer gill-plate. At some stage they are 

 fertilised by spermatozoa drawn in by the water currents, 

 though it is difficult to believe that this is entirely a matter 

 of chance. Development takes place within the external 

 gill-plate, and the larvse feed for some time on mucus 

 secreted by the gill. 



Development and life-history. — The development of Anodonta 

 differs in certain details from that of most bivalves, perhaps in adapta- 

 tion to fresh -water conditions. Moreover, a temporary parasitism of 

 the larva has complicated the later stages. 



