13 



which encircle the anterior end of the branchial sac. The 

 anterior band forms a complete ciliated ring, but the 

 posterior is interrupted in the ventral and dorsal median 

 lines : its ends becoming continuous respectively with 

 the marginal folds of the endostyle (ventrally), and with 

 the front of the dorsal lamina, where before joining, they 

 bound a narrow triangular cavity lined by ciliated epi- 

 thelium, the epibranchial groove (see PI. II., fig. 7, above 

 d.l.). Behind the peribranchial bands the proper wall of 

 the branchial sac commences. 



Branchial Sac and Atrium. 



The wall of the branchial sac is penetrated by a large 

 number of channels, through which blood flows. Some of 

 these run in one direction and some in another, so as to 

 form complicated but perfectly definite networks, which 

 differ in their arrangement in different kinds of Ascidians. 

 Between these blood channels there are clefts (the second- 

 ary gill-slits or "stigmata") in the wall of the branchial 

 sac, by means of which the water from the interior passes 

 into the large external or peribranchial cavity — the atrium. 



The transverse section (PI. II., fig. 2) shows how this 

 atrium surrounds the branchial sac on all sides except the 

 ventral, where the wall of the branchial sac becomes 

 contmuous with the body-wall. The right and left halves 

 of the atrium may be called the right and left peribranchial 

 cavities (p.br.). They unite along the dorsal edge to form 

 the cloaca, and there open to the exterior. The cavity 

 of the branchial sac communicates with the surrounding 

 atrium by means of the stigmata, as shown on the upper 

 half (left side) of fig. 2 (PI. II.). The section on the 

 right side is shown passing along a transverse vessel 

 between two of the rows of stigmata. 



