ASCIDIA CEASSA. <>1 



and terminate before the bottom of the sac is reached; 

 they are largest in the vicinity of the mouth. The 

 tentacular filaments at the base of the branchial tube 

 are about fifty, arranged in a single line on the pos- 

 terior surface of a muscular ridge or collar ; they are 

 rather closely set, moderately long and stout, but vary 

 in size. The branchial tubercle (PL XX, fig. 5) is 

 loop-formed, with the extremities turned first inwards, 

 then upwards. 



The ovary is placed in the intestinal loop, and 

 appears at the left side as a congeries of lobules. The 

 male caeca were not observed; the vas deferent, how- 

 ever, was followed into the loop of the intestine, and 

 the minute branches were traced amidst the cellular 

 matter which coats the alimentary canal, but the caeca, 

 probably from their minute size, escaped detection. 

 Both sides of the visceral mass are densely covered 

 with cellular matter ; the cells are minute and do not 

 exhibit the usual dark nucleus when viewed with a 

 doublet. 



Ascidia cmssa cannot very well be confounded with 

 any other known species ; the broadly oval form, the 

 sessile orifices, and hard, thick, cartilaginous test are 

 very characteristic ; but the branchial sac presents 

 probably the most distinctive feature. The walls of 

 this organ have a very peculiar appearance, on account 

 of the grooves of the minute plications being formed 

 into series of well-defined minute pouches by the 

 closely-set primary vessels, thus rendering it very 

 difficult to observe the secondarv vessels and the mi- 



f 



usually diminutive, elliptic stomata, And this diffi- 

 culty is increased by the excessive development of the 

 external vessels in connexion with the primary system 

 of blood-channels and the branchial ties, forming, as 

 they do, a complete network at the outside of the 

 branchial sac. It is only through the contracted 

 meshes of this network, or through the narrow 

 pouches on the inner surface, that the secondary 

 vessels and the stomata can be seen. 



