146 



MOLLUSCA 



excreta from the blood. Another excretory organ is the pericardial 

 gland or " Keber's organ." It lies just in front of the pericardium 

 and discharges into it. It may collect from the kidney; it secretes 

 uric acid. 



Reproduction. — The sexes are usually separate. A few forms 

 are hermaphroditic and protandrous. The gonads^ situated above 

 the foot, are paired masses of tubes which open just in front of the 

 renal aperture on each side. Spermatozoa pass out of the dorsal 

 siphon of the male and into the ventral siphon of the female. The 

 eggs pass out of the genital aperture and lie in the gills. They are 

 fertilized there and develop in a modified pouch or marsupium. 

 The eggs develop by cell division until in the fresh water mussel a 



Byssus 



SheU ^:".iA; 



Adductor muscle 



tfM^JfD 



Fig. 62. Glochldium of Anodonta. — A, a young mussel or glochidium. (After 

 Balfour.) B, the gills of a fish in which are embedded many young mussels forming 

 "blackheads." (After Lefevre and Curtis.) (From Hegner, College Zoology. 

 Courtesy of Macmillan Co.) 



glochidium is produced. This has a shell with two valves, hooked 

 in some species, and closed by muscles. The glochidia attach to 

 the gills or fins of a fish, become surrounded by the stimulated 

 epithelium of the host, and develop there until able to carry on 

 independent existence. They are thus dispersed by the fishes to 

 considerable distance. (Figures 62 and ^^?) 



In Venus and many other molluscs, we find that the embryo 

 develops as a free swimming larva of the trochophore type called a 

 veliger. 



Eggs. — Lamellibranchs have a large number of eggs. The 

 oyster has 300,000 to 60,000,000 per annum. The fresh water 

 mussel has 200,000. 



Nervous System. — There are two cerebral ganglia (one on each 



