334 



ZOOLOGY. 



Fig. 173. A QLOCHIDIUM. 



scopically. They are found in separate individuals. The 

 ova and spermatozoa when ripe pass into the supra - 

 branchial chamber ; in the case of the male, the spermatozoa 

 pass out into the water, and by 

 good luck some of them may be 

 carried into the mantle-chamber 

 of a female. Here the ova are 

 fertilized, and here more pre- 

 cisely, between the lamellae of the 

 outer gills they remain until they 

 have reached a larval stage of 

 development known as the glo- 

 chidium (fig. 173). This larva 

 has a bivalved shell, very different 

 in shape from that of the adult ; 

 there is only one adductor muscle, and the intestine is 

 spirally coiled. Each valve has large recurved teeth, and 

 an epidermal gland secretes a long adhesive filament, the 

 byssus. The glochidia are retained in the " brood pouches" 

 (the outer gills) until a stickleback or other suitable fish 

 approaches the mother. They are then discharged into the 

 water, and some may attach themselves to the fish, first by 

 the byssus and afterwards by the valve-teeth. The irritation 

 set up leads to a growth of the skin of the fish around 

 the glochidium, which, thus protected, goes through a 

 metamorphosis, and when it drops out it is a miniature 

 adult. This temporary attachment .to an actively swimming 

 fish secures the distribution of the young mussels up stream 

 as well as down, and insures them to some extent against 

 the danger of being washed out to sea a danger to which 



all fluviatile larvae are liable. 



I 



10. Classification. The mussel is a type of the 

 phylum Mollusca, characterized among other things by 

 the ventral muscular foot, the mantle-chamber with its 

 gills, the dorsal systemic heart, reduced coelom, ganglionic 

 nervous system, and absence of metamerism. It belongs 

 to the class Lamellibranchia, or Pelecypoda, which also 

 includes the oyster, cockle, scallop, and similar forms with 

 bivalved shells. The study of its structure would be much 



