20 



BRITISH TTJNICATA. 



spirals composed of minute secondary vessels. Thus 

 each spiral coil is set, as it were, in a square frame, 

 and forms a slightly-conical eminence which projects a 

 little into the branchial cavity. A few radiating vessels 

 pass from the apex of each cone to the points where 

 the transverse channels are intersected by the secon- 

 dary rectilinear vessels. Thus there are usually four 

 such radiating vessels to each coil. The longitudinal 

 bars are rigid and cord-like ; they project considerably 

 from the surface of the gill. The papillary membranes 



FIG. 30. Part of the branchial sac of Corolla parallelogram/ma. Much 



enlarged. 



are ample, extending from bar to bar, and have the 

 free margin considerably thickened, as is the case in 

 Axcidia scalra. In Corella, as in Ascidia, there are 

 no branchial papillse properly so-called ; but the thick- 

 ened edge of the membrane projecting from the bars 

 occasionally becomes assimilated to papillae. 



The oral lamina is replaced by a series of well- 

 developed filamentous processes, w r hich are wide at 

 the base and taper to fine points; they extend from 

 the top of the branchial sac and terminate near its 



