I'JOT.J NATURAL SCIENCES OF I'lII LADKLTII I A. 347 



terior lip, and it is not circular but is compressed laterally, as shown in 

 fig. 30. 



The blastopore then narrows until it is almost if not entirely dosed 

 (figs. 31, 32), and an invagination of ectoderm cells occurs here, which 

 is the stomodieum. The mt>uth is formed at the very place where the 

 blastopore closes, and the oesophagus is formed by the elongation of the 

 stomodeal invagination (fig. 36, (E.). 



In the closure of the blastopore and the formation of the mouth and 

 cesophagus Fulgur completely resembles Crepidula. 



7. INTESTINE AND OTHER PORTIONS OF THE ALIMENTARY CANAL. 



The intestine is formed from enteroblast cells derived from the 

 mesentomere 4f/. These enteroblasts lie in the midline behind the 

 shell gland and they constitute the stem of a Y-shaped group of cells, 

 the branches of the Y being the mesodermal bands (figs. 22, 23). In 

 its earliest stages the intestine is a round, densely staining group of 

 cells, which lies at a lower level than surrounding portions of the 

 blastoderm. After the posterior margin of the blastoderm has ex- 

 tended over to the ventral side of the embryo, the intestine lies between 

 the margin of the shell gland behind and the visceral nerve loop and 

 abdominal ganglion in front (figs. 28-30). 



About the time of the closure of the blastopore the anlage of the 

 intestine elongates toward the right (left in ventral view), becoming at 

 first eUiptical in shape (fig. 31) and then tubular (fig. 32 et seq.). Up 

 to the time of this elongation it lies in the median plane; after this it, 

 together Avith other organs posterior to the foot, moves to the right. 

 The end of the intestine which remains nearest the midline is the anal 

 end, though the anus does not form until a much later period; the end 

 farthest to the right is the gastral end. The intestine lies just in front 

 of and parallel with the edge of the shell ; in fig. 32 its course is nearly 

 transverse to the long axis of the embryo, the notch in the shell being 

 at this stage very shallow. As the shell grow"s forward on the dorsal 

 side more rapidly than on the ventral, this notch grow\s deeper, and the 

 right edge of the shell becomes parallel v.ith the long axis of the embryo 

 (figs. 33-35) ; at the same time the intestine turns from a transverse to 

 a longitudinal course. The visceral nerve loop accompanies the in- 

 testine in this movement, the abdominal ganglion always lying close 

 to the mesial side of the intestine. 



Finally the mantle cavity begins to form on the mesial side of the 

 intestine, and the latter is thus carried into the roof of the mantle 

 chamber, the anal end of the intestine remaining near the mantle 



