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a rather advanced stage of development, when they are extruded as minute, free- 

 swimming bivalve larvae, to be distributed far and wide by tides and currents. 



According to Hatscheck (1881), who has studied the larval development of 

 Teredo in greater detail than any one else, the larvae enter the gills of the parent from 

 the posterior end. so that the most advanced embryos are found in the anterior portion 

 of the gills. Exactly what occurs in this connection has never been found out. The 

 embryos are probably nourished from the blood of the parent, of which the gills 

 have an abimdant supply. 



In some species of Teredo, such as 7". norvegka, and in all forms of Bankia of 

 which the breeding habits are at all known, no brood pouch is developed, but the 

 eggs apparently are extruded into the water, where they depend on chance fertiliza- 

 tion, and the larvae during their entire development are subjected to the vicissitudes 

 of pelagic life. This must mean a greatly increased coefiicient of fatalities among the 

 larvae, any individual egg having a much slighter chance of fertilization, and still 

 less of reaching maturity, than is the case in the forms which retain the developing 

 larvae in the gills. This doubtless explains the fact that Bankia is ordinarily less 

 numerous than the incubatory species of Teredo. 



