SECT, xii RELATION OF APUS TO CRUSTACEA 205 



thorax. In Apus we have a more primitive stage, 

 in that the livers are still clearly little more than 

 digesting clivcrticula of the mid-gut, at whose branched 

 ends only are found the hepatic glands. In Limulus, 

 the glandular portion is far more pronounced, and the 

 diverticula themselves arc diminished to bile ducts, as 

 is the case in the higher Crustacea. 



The very difference between what we find here and 

 in Apus is instructive ; perhaps, from the fact of there 

 being two ducts on each side, we can conclude that 

 there were originally two or more intestinal diverti- 

 cula in Apus. The general form of the liver of Apus 

 certainly looks as if it consisted of two or more diver- 

 ticula run together at the places where they open into 

 the mid-gut. Embryologically (according to Packard^ 

 the livers of Limulus begin as simple biliary tubes, 

 the branchings following later. The development of 

 the liver as outgrowths of the mid-gut is well shown 

 in Claus' figures of the Nauplius (Figs. 39, 41). 



The rest of the alimentary canal offers nothing 

 special for remark ; like that of Apus it has a short 

 rectum, the chitinous intima of which is thrown into 

 longitudinal folds by the musculature. It is worth 

 noting that, whereas the anterior half of the mid-gut 

 is very thin-walled, it gradually gets thicker and 

 more muscular as it approaches the rectum ; there is 

 no sharp division between the two. This is exactly 

 what we found in Apus. 



The circulatory system of Limulus is very highly 

 specialised. We do not, as already stated in Part I., 

 lay much value upon it from a morphological point 



