BIONOMIC CHARACTERS OF MOLLUSCA 1017 



The marine pelecypod normally passes through a mero-plank- 

 tonic larval stage — the trochophore — in which the young is pro- 

 vided with a velum, furnished with vibratory cilige (veliger stage). 

 At certain seasons of the year these ciliated embryos swarm in the 

 pelagic district, especially in the neighborhood of the shores, where 

 they become the sport of the currents, which distribute them far 

 and wide. When they finally settle down upon the sea bottom, on 

 the loss of the velum, they develop further if they reach the proper 

 substratum, other conditions being favorable. Vast numbers of 

 the larvae are destroyed before they reach the bottom, serving as 

 food for all kinds of animals, or succumbing to unfavorable condi- 

 tions; and vast numbers of others die from falling on an unfav- 

 orable bottom. That most species nevertheless develop to the 

 fullest extent is due to the enormous fecundity of most pelecypods. 

 As an extreme example may perhaps be mentioned our common 

 northern oyster, Ostrea virginiana, which, according to Brooks 

 (2:xxviii), produces nine millions of eggs. In fresh- water pelecy- 

 pods the mero-planktonic veliger larva exists in one species only 

 {Drcissensia polymorpha), which is believed to have migrated from 

 salt to fresh water in recent geologic* times. (Lang-19.) In the 

 other freshwater pelecypods the development proceeds in a different 

 manner, special adaptations to special modes of life being met with. 

 In some cases (Pisidium, Cyclas) the eggs develop in special brood- 

 capsules in the gills of the mother, which they leave with shell 

 fully formed, as young bivalves. In these genera the velum re- 

 mains rudimentary, the animal passing through the trochophore 

 stage within the gills of the mother. In the Unionidas the embryo 

 passes through its several stages in the gill of the mother, leaving 

 it with a bivalve shell, which is, however, furnished with a trian- 

 gular process on the ventral border of each valve, by means of 

 which the embryo attaches itself to the fins (Anodonta) or gills 

 (Unio) of fishes. In this manner the animal leads an epinektonic 

 existence, becoming enclosed by the rapid growth of the epithelium 

 of the part where the embryo is attached, and leading then a truly 

 endo-parasitic life. After several weeks the embryo has become 

 transformed into a young mussel, which, breaking through the 

 enclosing tissue of its nest, falls to the bottom of the water, there 

 to develop into the adult. 



ScApiiopoDA AND Amphineura. The first of these classes is 

 represented by the Dentalidse ; the second by the Chitonidas, which 

 alone are important paloeontologically. Both are marine, being of 

 a sedentary benthonic habit, though not permanently attached. 

 Dentalium lies buried in the mud and sands usually at great depths. 



