MORPHOLOGY OF CYCLOSTOMATA. 357 



The peritoneal membrane lining tlie enteric canal immediately 

 behind the branchial region is also thicker as compared witli 

 other parts (fig. 107), being composed of a single layer of cubical 

 cells, — a ^peculiarity observed since the last stage (compare figs. 

 98-90 with fig. 107). 



The pronephric tubules as well as the collecting duct are 

 composed of a regular epithelium of cylindrical cells ; the former, 

 moreover, are much prolonged and, in some parts (fig. 108), much 

 coiled, so that the peritoneal cavity which was almost a liollow 

 space in the last stage, is filled up with the tubules and the 

 cardiac tube. 



Fig. 107 represents the section through the hind part of the 

 sixth myotome; a pair of the tubules {pt.3) is hanging down in 

 the body-cavity immediately behind the hind wall of the branchial 

 chamber. On the right side, the axial plane of the tubule is 

 cut through, while, on the left, the anterior wall of it is sliced ; 

 these are the tubules of the third pair. They show no bending 

 in the antero-posterior direction, but are curved laterally and ven- 

 trally. The component cells are, in the nephrostomal portion, 

 taller in comparison with those in other parts of the tubule or the 

 collecting duct. The fourth tubule and nephrostome are seen on 

 the right side of fig. 108, while on its left side, the communication of 

 the corresponding tulnile on the opposite side with the collecting 

 duct is recognizable. The left ne2:)hrostome is found in the third 

 section behind this, which is not figured. This pair of the tubules 

 exhibits, in section, constrictions at two or three points owing 

 to their curving somewhat in the antero-posterior direction (see the 

 tubule on the right side of fig. 108). Fig. 109 is the section im- 

 mediately behind the last and shows the cross-sections of the 

 collecting duct {cd.) and a piece of the left fourth tubule {j)t.4). 



