MOLLUSCA. 285 



always developed 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, (i) A stomo- 

 dgeum, formed by an epiblastic invagination, which gives rise to 

 the mouth, oesophagus and salivary glands. (2) A proctodseum, 

 which is an extremely small epiblastic invagination. (3) A 

 mesenteron, lined by true hypoblast, which forms the main 



c7is 



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 ; oc. oesophagus ; ds. yolk-sack ; chs. 

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

 of the funnel. 



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. I r I 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 hypoblast and epiblast there is a very well marked 

 layer of mesoblast. As the mesenteric cavity extends, its walls 



1 The following description applies specially to Loligo. 



