ANATOMY AND PHYSIOLOGV. 47 



disposed in regular traiisverse series between the transverse 

 channels. The coiled vessels do not so frequently intercom- 

 municate as they do \nMolgida; consequently the stigmata 

 are much longer, being less interrupted. The radiating 

 vessels are not numerous, and they pass from the centre of. 

 the coil to the transverse channels. The longitudinal bars are 

 more rigid and cord-like than usual ; they project considerably 

 from the surface of the gill ; and the papillfe which are at- 

 tached to them are not elevated, but inclining backwards are 

 united throughout their length, and give support to wide 

 membranes that extend from the surface of the transverse 

 vessels. In this species the oral lamina is replaced by a series 

 of well-developed filamentous processes. 



The most perfect, however, of the spiral type of gill is found 

 in Molgula areiiosa. In this interesting species the interior of 

 the branchial sac is furnished 011 each side with six or seven 

 wide, longitudinal, ribbon-like bands, which are attached by 



FIG. 7. Tentacles of Ascidia Alderi. Highly magnified. 



one edge to the ti-ansverse vessels at the points where they 

 cross them. These bands, how like soever in general appear- 

 ance, are not to be confounded with the branchial folds in 

 Stye-la and Molrjula ; they are the homologues of the longitu- 

 dinal bars so constantly present, and with the transverse 

 vessels give to the surface a coarse reticulation, the square 

 meshes of which are each occupied by a conical eminence. On 

 account of the prominence of the longitudinal bands these 

 eminences, which look like so many miniature beehives, seem 

 to be arranged in six or seven longitudinal series. Each cone 

 is formed of a double spiral coil of secondary vessels united at 

 the apex ; the coils are .perfect, and the stigmata, which are 

 coextensive with them, appear to be scarcely, if at all, inter- 

 rupted by intervascular communications. Radiating vessels, 

 however, which are sufficiently numerous to prevent engorge- 

 ment, pass from the apex of each cone to the transverse 

 vessels, and are the principal interruptions to the con- 

 tinuity of the spiral stigmata. The oral lamina in this, as in all 

 the members of the genus, is a plain simple membranous band. 



