18 



(27) observed, (3) a thin basement lining, (4) an epithelial 

 layer, Avhich is somewhat columnar just behind the 

 stomach, where it is lined by a thin cuticula. Further 

 back, the epithelium rapidly becomes flatter, and loses its 

 chitinous lining in the fourth segment of the thorax. In 

 the sixth thoracic segment the dorsal epithelium again 

 becomes columnar, and the basement lining appears dis- 

 tinctly cuticularised, projecting inwards betAveen the cells 

 of the epithelium so as to form a kind of palisade. From 

 the first segment of the abdomen backwards the epithelium 

 is distinctly columnar and stains deeply, the chitinous 

 basement palisade being strongly marked. In the third 

 segment of the abdomen the dorsal epithelium becomes 

 thicker and is drawn out laterally ; in the next segment 

 it becomes prolonged at the sides into a pair of diverticula 

 which run forwards, side by side, above the mid-gut, as 

 far as the sixth thoracic segment (PI. II., div. p.). That 

 these posterior diverticula arise from the mid-gut as 

 pointed out by Wrzeniowski (27) and K"ebeski (14) and 

 not from the hind-gut, as Sars (19) supposed, is clear 

 from the fact that their epithelium is gradually differ- 

 entiated from, and closely resembles, that of the mid-gut, 

 whilst a distinct break occurs between their epithelium 

 and that of the hind-gut, which joins the mid-gut imme- 

 diately behind their point of origin. The epithelium of 

 the posterior diverticula is composed of very tall columnar 

 cells, with deeply staining nuclei at their bases, which 

 were found by Spencer (21) to contain phosphoric acid 

 in the case of Talitrus locusta. They are surrounded by 

 thin connective tissue, which separates them from the 

 posterior aorta, in Avhich they apparently lie (PI. II., div. 

 p.). Their lumina are constricted dorso-veutrally though 

 wide from side to side, and they have no intima. Between 

 the mid- and hind-gut, depending from the dorsal wall of 



