236 



A L IMENTA R Y TEA CT. 



as a very shallow epiblastic invagination, which does not give rise to 

 any part of the true intestine. 



In the Cephalopods the alimentary tract is formed, as in other 

 cephalophorous Mollusca, of three sections. (1) A stomodseum, formed 

 by an epiblastic invagination, which gives rise to the mouth, oesophagus 

 and salivary glands. (2) A proctodaeuin, which is an extremely small 

 epiblastic invagination. (3) A mesenteron, lined by true hypoblast, 



c7is 



it 



oe 



~brd, 



FIG. 126. LONGITUDINAL VERTICAL SECTION THROUGH A LOLIGO OVUM WHEN THE 



MESENTERIC CAVITY IS JUST COMMENCING TO BE FORMED. (After Bobretzky.) 



gls. salivary gland ; brd. sheath of radula ; oe. oesophagus; da. yolk-sack; chs. shell- 

 gland; mt. mantle; pdh. mesenteron; x. epiblastic thickening between the folds 

 of the funnel. 



which forms the main section of the alimentary tract, viz. the stomach, 

 intestine, the liver, and ink sack 1 . 



The mesenteron. The mesenteron is first visible from the surface 

 as a small tubercle on the posterior side of the mantle between the 

 rudiments of the two gills (fig. Ill B, an). Within this, as was first 

 shewn by Lankester, a cavity appears. 



This cavity is as in Gasteropods open to the yolk sack, and only 

 separated from the yolk itself by the yolk membrane already 

 spoken of. It is at first lined by indifferent cells of the lower 

 layer of the blastoderm, which however soon become columnar and 

 form a definite hypoblastic layer (fig. 126, pdh}. Between the hypo- 

 blast and epiblast there is a very well marked layer of mesoblast. 

 As the mesenteric cavity extends, its walls meet the epiblast, and 

 at the point of contact of the two layers the epiblast becomes slightly 

 pitted in. At this point the anus is formed at a considerably 

 later period (fig. 127, an). 



On the ventral side of the primitive mesenteron an outgrowth 

 appears very early, which becomes the ink sack (fig. 127, bi). 



The mesenteric cavity, still open to the yolk, gradually extends 

 itself in a dorsal direction over the yolk sack, but remains for some 



1 The following description applies specially to Loligo. 



