32 THE SCALLOP FISHERY 



leading into a cavity in the center of the body, the stomach, and an 

 abbreviated intestine with a posterior anal opening; (4) an inconspicu- 

 ous mantle; (5) anterior adductor muscle alone present; (6) size .093 

 millimeter. The increase in size from the trochosphere stage is due to 

 the formation (Fig. 16) of a cavity between the body and the shell. 



The late veUger is characterized by: (1) a shell of the same structure, 

 marked by prominent umbones directed posteriorly; (2) a well-devel- 

 oped foot which has succeeded a degenerated velum as the. swimming 

 organ; (3) a more complex digestive tract, with palps, and a coiled 

 intestine; (4) a conspicuous mantle; (5) a posterior adductor muscle, 

 and the appearance of several gill bars; (6) size, .18 millimeter. 



In studying the life history of nearly every large lamellibranch which 

 begins its life external to the parent, there is a gap between the ana- 

 tomical changes of the early and late veliger periods, as it is a difficult 

 stage to procure specimens for study. It is only possible in this history 

 of the scallop to give the changes in the different organs by comparing 

 the early and late veligers, as we have not been able to identify with 

 certainty the intermediate forms on account of the large number of 

 species which so closely resemble each other, as they are collected in 

 the plankton net at the surface. 



The Shell. — The veliger shell of the Pelecypoda or lamellibranchiate 

 moUusks has been aptly given the name prodissoeonch by Jackson (4) 

 to distinguish it from the succeeding shell, the dissoconch. With the 

 scallop, I have taken the liberty to apply this term, which properly 

 includes all of the veliger stage, to merely the late veliger, at which time 

 it has acquired a form markedly characteristic of the scallop. Here- 

 after, when speaking of the prodissoeonch shell, it refers only to the 

 form of shell typical of the late veliger, as it remains differentiated 

 from the succeeding dissoconch stage. In the early veliger, the shell 

 consists of two valves of homogeneous structure joined dorsally by a 

 ligament ia a slightly concave hinge-line. 



The change from the flat hinge veliger (Fig. 17) to the completed 

 prodissoeonch (Fig. 18), which marks the end of the veliger stage, is 

 quite pronounced. The straight hinge line has given way to one of 

 slight curvature, while the valves by their gro'W'th have formed promi- 

 nent umbones, hiding the hinge line from lateral view. The umbones 

 point posteriorly, but are less prominent than in the ease of the oyster. 

 The left valve is more convex than the right, and the right umbo is less 

 prominent than the left (Fig. 19). In the completed prodissoeonch and 

 probably in the early veliger ten pairs of teeth can be seen along the 

 hinge line, five on each side of a central slit (Fig. 22). The question of 

 teeth has always been of interest in the classification of lamellibranchs. 

 These are later either obscured or absorbed by the growth of the shell. 

 The teeth of one valve fit into the depressions of the other, adding 

 strength to the hinge. The shell remains homogeneous, except for fine 

 lines of growth parallel to the free edge. Its calcareous composition is 



