334 EENEST AVARREN. 



layer of connective tissue and a somewhat peculiar ectodermal 

 epithelium. Anteriorly the cuticle lining is more or less 

 smooth (PI. XVIII) and resembles that of the sucking' 

 pharynx ; posteriorly the cuticle is thick and almost fibrous 

 in character, it is not sharply delimited from the epithelium 

 itself, and it is raised into prominent, pointed spines {sjn.). 

 These spines are very long and numerous and extend over 

 the posterior two-thirds of the oesophagus ; they must consti- 

 tute not only a very efficient strainer, but must serve to 

 commingle the food mass as it is forced through it. 



The oesophageal epithelium is invaginated into the front 

 end of the mesenteron to form a small but quite definite 

 oesophageal valve consisting of tall cells. This invagination 

 involves the thin connective tissue but not the muscle-layer 

 of the e"ut-wall. The valve forms an obvious constriction 

 between the oesophagus and mesenteron and doubtless the 

 passage can be entirely closed in this plane (PI. XYIII, 

 a\ v.). 



Mesenteron. — This portion of the gut in Par ac or o toe a 

 is very large, not to say hypertrophied (PI. XVIII). It 

 extends from the metathorax to the region of abdominal 

 segment VII. In the ordinary position of the abdomen the 

 main portiou is V-shaped with the two limbs approximately 

 equal. At the posterior end (morphological) the mesenteron 

 is sharply bent on itself dorsally, it narrows greatly and joins 

 the ileum of the proctodeum in the region of segment VI. 

 The endodermal epithelium of the mesenteron consists of 

 cells of very irregular height giving a conspicuously waved 

 inner surface (PI. XIX, fig. 59, e.). Many of the cells possess 

 large clear oval vacuoles of the usual type. In transverse 

 section the wall of the mesenteron may show considerable 

 longitudinal folding (fig. 60). Wedged at the base of the 

 columnar cells there are small deeply-staining cells [gl. c.) 

 resembling the cells which secrete HCl in the gastric gland 

 of a mammal. It appears that the vacuolated cells are some- 

 times bodily discharged into the food-mass, since a number 

 were found embedded in it iu a good series of sections. On 



