386 PROCEEDINGS OF THE ACADEMY OF [April, 



regions. As development proceeds it is elongated, its posterior end 

 being ventrally directed and turned toward the right. The develop- 

 ment of the liver of Crepidula comes later, being retarded by the 

 great amoimt of yolk. 



The next change in the development of the enteron of Fiona may 

 be observed in fig. 105, which represents a veliger in which the ali- 

 mentary canal is beginning to become differentiated into several parts. 

 Anteriorly is seen the stomodseiim, which has as yet not broken through 

 but touches the wall of the enteron. Above and to the left of this 

 point of contact is a decided lobing of the wall of the enteric cavity, 

 formed of the large yolk-ladened cells which at an earlier period lay in 

 the anterior region of the archenteron. This is the rudiment of the 

 liver, and as development proceeds the invagination becomes larger and 

 more constricted at its base, forming a rounded lobe upon the left dorsal 

 wall of the enteric canal. Behind the rudiment of the liver the enteron 

 has widened into a capacious sac which is larger at its upper anterior 

 end, the walls of the whole being formed of rather small cells which are 

 yet rich in yolk. This is the stomach, and it ends blindly against the 

 intestinal mass behind and to the right. The intestine is yet a solid 

 strand of cells connecting the posterior end of the stomach with the 

 ectoderm. With the growth of the veliger this strand has become 

 more slender, elongated and turned forward, its distal end lying well 

 up on the side of the body behind the constriction which forms the 

 foot. The huge excretory cell lies just dorsal to this point (figs. 106, 

 107). In figs. 90, 91, 92 and 93, which represent coronal sections of 

 a veliger somewhat older than figs. 105 and 106, and shghtly more 

 mature than that of fig. 107, it will be seen that the intestine is still a 

 solid strand of cells; and that the oesophagus is as yet not in open con- 

 nection with the rest of the alimentary canal. An examination of a 

 considerably older larva (figs. 109, 110) shows a very small lumen, 

 just beginning to form in the center of the intestinal strand, but as 

 yet no communication between oesophagus and enteric cavity. 



Stomodceum and Mouth. 



As the blastopore narrows (fig. 79) it becomes entirely surrounded, 

 except at the anterior end, by third quartet cells. At the anterior 

 point second quartet cells from 2b" and 2b-^^ lie along the edge also. 

 Figures of a later stage (as 97, 98) show the blastopore as a mere 

 rounded opening, its edges and walls below thickly set with darkly 

 nucleated cells, and when complete closure occurs a plug of these cells 



