70 BRITISH TUXICATA. 



channels are placed at regular intervals and scarcely 

 vary at all in size ; and between and opening into 

 them at right angles are numerous small longitudinal 

 secondary vessels divided by elongated spaces or 

 stomata, so that the whole forms a reticulation of 

 vessels, in which the transverse channels are lar'e and 



J o 



distant, the longitudinal small and numerous, and 

 divided only by narrow open spaces (Plate X). 



This simple structure of the aerating organ is not 

 usual in the Avcidiw, though it is fundamentally the 

 same in all. In A. mentida, A. sordida, A. scnbrn, A. 

 ((ffiitis, and their allies, the simple reticulated branchial 

 web of A. reuoxa has, as it were, become minutely 

 folded or plaited longitudinally, so as to give to the 

 surface the appearance of corduroy, but the flutes or 

 grooves between the ridges or plaits are interrupted 

 wherever the transverse vessels cross them, turning 

 them into series of minute recesses or pouches. 



The longitudinal bars which project from the true 

 aerating surface of the branchial sac extend the whole 

 length of the organ ; they are usually stout and rather 

 rigid, and are attached only at the points where they 

 cross the primary transverse vessels, and at this point, 

 too, they have projecting from their upper surface 

 elongated papilla? which usually bear ciliated discs, 

 and have a wide membrane stretched from the back 

 for a considerable way along the transverse vessels. 



t/ 



In A. mentula and allied species there are small inter- 

 mediate papillae between those at the points of inter- 

 section. In A. sonliilii, A. xi-aLi-n, and A. ft nix there 

 are no intermediate papilla? on the bars, but in all of 

 them the papillae at the points of attachment are in 

 connection with a wide membranous expansion, and 

 distinct ciliated discs are arranged in pairs on the 

 transverse vessels between the longitudinal liars. In 

 some species there is a ciliated disc on the liars 

 between the papillae. In A. venosa the bars are more 

 delicate than usual, and the membrane in connection 

 with the papillae is inconspicuous. 



