354 



passing- through the Trochophore stage within the gills of the 

 mother. In the Unionid^ the embryo passes through its 

 several stages in the gill of the mother, leaving it Avith a 

 bivalve shell, which is, however, furnished with a triangular 

 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 a 

 pseudo-nektonic existence, becoming enclosed by the rapid 

 growth of the epithelium of the part where the embryo is 

 attached, and leading thus a truly endo-parasitic life. After 

 several w^eeks 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. 



Scaphopoda and Amphineura. — The first of these classes 

 is represented by the Dentalidse, the second by the Chitonida', 

 which alone are important pateontologically. 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, while Chiton and its 

 allies cling to stones, shells, etc., and are rare in deep water, 

 where only their more archaic representatives occur. A few 

 species of Dentalium occur in moderately shalh^w water, 

 but most of them live below the hundred-fathom line, some 

 reaching a depth of 2,000 fathoms or more. Chiton seldom 

 extends beloAV 500 fathoms. In Itoth grou|)S a mero-plank- 

 tonic larva occurs. 



(rastro])oda. — The gastropods are typical benthonic ani- 

 mals, inhabiting the sea, fresh water, and the land. They 

 almost invariably belong to the vagrant benthos, though 

 the degree of locomotion varies greatly among different 

 species. Among the exceptions to the general vagrant 

 habit, are Vermetus and some other genera, which live a 

 truly sedentary lienthonic life, being attached to rocks or 

 shells. Some genera, like Capulus, adhere continually to 

 shells and the tests of Echinoderms and Crustacea; while the 

 limpets, though adhering ])0\verfully to rocks and shells by 



