264 



LAMELLIBRANCHIATA. 



make their appearance, derived in the main from the epiblast, though 

 probably in part also from the hypoblastic vesicle (vide fig. 119 C, p). The 

 cavity between the hypoblast and epiblast, which contains these cells, is the 

 body cavity. Fig. 119 represents three views of the embryo at this stage. 

 A is a surface view shewing the epiblast ; B is an optical section through 

 the median plane shewing the hypoblast and some of the mesoblast cells ; 

 and C is an optical section shewing the mesoblast cells. A prominence on 

 one side of the embryo now develops which forms the commencement of the 

 foot, and the archenteric sack grows out at its free extremity into two lobes, 

 but remains attached to the epiblast by an imperforate pedicle. The next 

 organ to appear is the stomodseum. It arises as a ciliated epiblastic in- 

 growth which meets the free end of the archenteric sack, fuses with it, and 

 shortly afterwards opens into it (fig. 118, pK). Between the mouth and the 

 attachment of the enteric pedicle is placed the foot (/), which becomes 

 ciliated. On the dorsal side of the enteric pedicle there appears a saddle- 

 shaped patch of epiblast cells bounding the sides of a groove (shs). This is 

 the rudiment of the shell-gland. 



The enteric pedicle, or intestine as it may now be called, soon acquires a 

 lumen, though still imperforate at its termination where the anus is 

 eventually formed. Ventral to the intestine is placed a mass of cells the 

 rudiment of the organ of Bojanus. It is stated to be developed as an 

 ingrowth of the epiblast. 



In a slightly later stage the shell-gland rapidly increases in size and 

 flattens out, and on the two sides of it there appear the rudiments of the two 

 valves, which are at first quite distinct, and separated by a considerable 

 interval (fig. 120). Before the appearance of the valves of the shell, the 

 mantle folds have already grown out from the sides of the body. 



At a somewhat later stage the gills 

 appear as a linear series of small inde- 

 pendent buds within the folds of the mantle 

 behind the foot (fig. 120, br). The ante- 

 rior adductor also becomes differentiated. 



The alimentary tract in the meantime 

 has undergone considerable changes. The 

 primitive lateral lobes dilate enormously 

 and become ciliated. At a still later stage 

 their walls undergo peculiar changes, the 

 nature of which is somewhat obscure, but 

 they appear to me to be of the same charac- 

 ter as those in many Pteropods and Gas- 

 teropods, where the cells of the hepatic 

 diverticula, to which the lobes of Pisidium 

 apparently correspond, become filled with 

 an albuminous material. 



The later stages in Pisidium have not 

 been followed. 



B 



FIG. 120. DIAGRAMMATIC VIEW 

 OF ADVANCED LARVA OF PlSIDIUM. 

 (Copied from Lankester.) 



m. mouth ; a. anus ; B. organ 

 of Bojanus ; mn. mantle ; f. foot. 



