91 



canal which is derived from the archenteron and is lined 

 by cells formed from the hypoblast. 



From the mesenteron arise a pair of caeca the 

 so-called " pyloric caeca." This is an unfortunate 

 designation, as they do not arise from the pyloric region 

 of the fore-gut. 1 therefore propose to substitute the 

 name of Mid-gut caeca (fig. 34, caec.}. Each caecum 

 arises from the side of the anterior part of the mid-gut. 

 It passes forward as a narrow tube alongside the pyloric 

 chamber, and is closely applied to the postero-lateral 

 region of the cardiac chamber of the fore-gut. On a level 

 with the widest part of the latter the caecum terminates 

 in a much convoluted portion. 



The digestive glands (fig. 34, di. gl.} arise at each 

 side from the ventro-lateral region of the mid-gut, 

 immediately behind the origin of the mid-gut caeca. 

 These will be described more fully below. 



The epithelium lining the mid-gut (PI. X, fig. 61) 

 consists of columnar cells having a length of 55 n in an 

 adult crab. There is no cuticular lining to the epithelium 

 of this region, but each cell has an outer striated border 

 from 1 fJL to 2 p in thickness. This is similar to the 

 " Harchensaum " present in the mid-gut of Anurida* and 

 in the duodenum of many vertebrates. It is very probable 

 that this striated border is characteristic of the mid-gut 

 epithelium of arthropods in general, and probably the 

 thin cuticle lining the mid-gut of Ligia described by 

 Hewittf is a similar structure. 



In many of the epithelial cells are refractive bodies, 

 probably the fat globules mentioned by Cuenot. Beneath 

 the epithelium is a thick basement membrane. In the 

 comparatively broad layer of connective tissue beneath 

 the basement membrane are thin layers of circular and 

 longitudinal muscles. 



* Imms. L.M.B.C. Memoir, " Anurida." 



t Hewitt. L.M.B.C. Memoir, " Ligfa." 



