becomes somewhat pointed behind ; that is, the end directed 

 away from the apical cilia becomes the pointed end. The embryo 

 enlarges, due to the formation of a space beneath the shell gland, 

 which has now become the mantle, and the surface cilia become 

 restricted to the anterior end. The archenteron begins to grow 

 rapidly, enlarges to form the stomach, (s.) and grows pos- 

 teriorly to form the intestine (i.). The anterior adductor muscle 

 (aam.) makes its appearance dorsal to the apical plate (ap.) 

 and posterior to the dorsal margin of the portion that bears 

 surface cilia, which later develops into the velum. The space 

 between the developing alimentary canal and the body wall is 

 quite extensive, practically surrounding the alimentary canal 

 except where it joins the body wall at each end and where the 

 anterior end of the stomach is in contact with the apical plate. 

 The adductor muscle is in contact with the body wall on its 

 anterior surface, but is otherwise surrounded by this space. A 

 few greatly elongated spindle-shaped fibers resembling muscle 

 fibers usually extend across the space. Almost universally one 

 or two such fibers extend from the dorsal surface of the stomach 

 dorsally and posteriorly to the body wall. Similar fibers have 

 been noticed in the embryos of other lamellibranchs and are 

 quite conspicuous in Nucula, (7) but their function is not 

 known. The space is no doubt a schizoccele that is formed as 

 the result of the arching up of the shell gland to form the 

 mantle. This takes place much more rapidly than the internal 

 organs grow, and the space is accordingly formed. Its ultimate 

 fate has not been traced as the oldest of the embryos reared still 

 have a remnant of it dorsal to the alimentary canal. 



The stage that has been described is practically a trochophore. 

 The cilia are in front of the mouth, but cover the whole area 

 around the apical tuft instead of being arranged to form a band. 

 Later, as the velum is formed, they are better developed along 

 the margins of its lobes and thus form a band. 



A stage similar to this is probably present in all forms of 

 lamellibranchs that do not give protection to their embryos as 

 is done by the Unionidae, Sphaerium, etc. Even here (32) 

 something that corresponds to the stage may be recognized. At 

 first sight the embryos of Yoldia (5) and Nucula (7) seem to 

 differ considerably from the trochophore that has been described, 



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