260 



THE RISE OF ANIMAL LIFE 



Ji # vooi^ dams 



0" 



F!g. 12-10. Life cycle of the clam. 



pair of ganglia is connected by a commis- 

 sure and by two connectives, the cerebro- 

 visceral and cerebropedal to the other 

 gangHonic pairs. Small nerves extend from 

 the ganglia to the body surface and the 

 muscles. Although the clam is a highly spe- 

 cialized animal in many respects, its nerv- 

 ous system is comparatively primitive. First 

 of all, there is little evidence of a brain. 

 Furthermore, the connectives that surround 

 the esophagus are highly reminiscent of the 

 circumpharyngeal connectives of the earth- 

 worm. The sensory apparatus is limited to 

 sensory cells on the siphon margins, tactile 

 organs on the mantle, and some areas which 

 are believed to be sensitive to chemicals. 

 Most of these structures resemble similar 

 parts of lower type animals. Clams are 

 slow, sluggish animals and the sensory sys- 

 tem required is relatively simple. 



Fresh- water clams, for the most part, are 

 dioecious, but some are hermaphroditic. 

 The ovaries and testes, yellow in color and 

 surrounding the intestine, constitute much 



of the visceral mass (Fig. 12-6). The 

 sperms of the male are liberated from the 

 testes through the genital pore, just ventral 

 to the aperture of the bladder portion of the 

 kidney. From here they are carried through 

 the body to be discharged through the ex- 

 current siphon. As water is carried into 

 the incurrent siphon of the female it may, 

 purely by chance, carry sperm cells with 

 it. These then enter the suprabranchial 

 chamber of the gills where the ova dis- 

 charged from the ovary await fertilization. 

 After fertilization, the eggs are drawn into 

 the water tubes of the gills and attached to 

 them by mucus. While the tubes are carry- 

 ing eggs they become enlarged and are 

 called brood chambers. After a period of 

 development, the zygotes become small 

 larval glochidia (singular, glochidium), 

 complete with two valves and a larval thread 

 (Fig. 12-10). Many species develop a 

 hooked valve. At this stage they are dis- 

 charged into the water through the excur- 

 rent siphon of the female where they may 



