10G 



BRITISH TUXICATA. 



towards the ventral margin. The anal tube is three- 

 fourths down, a little within the ventral margin ; it is 

 turned backwards and is so short that in the contracted 

 state it appears merely as an eminence. 



The test (fig. 21) is pellucid, thick, firm, and car- 

 tilao-inous, of a yellowish horn-colour and sparsely 



^^ * 



covered with minute, sharply-defined, soft, conical 

 papillae, most conspicuous in young specimens. The 

 blood-channels are not very much subdivided, and they 

 terminate in rounded but not enlarged extremities. 



The mantle is well supplied with delicate muscular 

 fibres, tapers gradually and is very narrow in front, 

 subsiding imperceptibly into the well-produced bran- 

 chial tube. The anal tube is more than three-fourths 



FIG. 21. Ascidia producta. Natural size. 



down the ventral margin, and is short and turned 

 backwards. 



The branchial sac (PI. XVII, fig. 9) is long and 

 narrow, tapering upwards, and extends very little 

 backwards beyond the visceral mass ; it is minutely 

 and somewhat obscurely plicated. The primary vessels 

 are rather closely set and variable in size, with a few 

 much larger than the rest at distant intervals. The 

 stomata are short and rectilinear, with the extremities 

 obtuse. The longitudinal bars are stout, with papillae 

 only at the intersections of the meshes; the papilla 

 are rather long and slightly shouldered, with the apices 

 pointed and a little produced. The papillary mem- 

 brane is obsolete. The oral lamina (PI. XIX, fig. 7) is 

 rather narrow, dies out a little below the mouth, and 

 is succeeded by a few minute transverse ridges which 



