STRUCTURE BRANCHIAL SAC 



47 



of the branchial sac to the oesophagus, guided by a membranous 

 fold, the dorsal lamina (Fig. 21, d.l), which is more or less ridged 

 or corrugated, and may be armed with marginal tags or even 

 replaced by larger processes (the " languets ") in some species 

 of Ascidians. In the living animal the lamina has its free 

 edge curved to the right hand side in such a manner as to 

 constitute a fairly perfect tube along which the train of food 

 passes. 



Branchial Sac. — Thus we have the dorsal lamina (or the 

 languets) along the dorsal edge, the endostyle along the ventral 

 edge, and the peripharyngeal bands around the anterior end. The 

 wall of the branchial sac itself is penetrated by a large number 

 of channels through 



which blood flows. Some 

 of these run in one direc- 

 tion and some in an- 

 other, so as to form 

 complicated networks, 

 which differ greatly in 

 their arrangement in 

 different Ascidians. Be- 

 tween these blood- 

 channels there are clefts 

 (" stigmata "), the 

 secondary or subdivided 

 gill-slits, by means of 

 which the current of 

 water passes from the 

 branchial sac to the 

 large external peribran- 

 chial or atrial cavity. 



ggs;^ g-""'"~Sph. 



>br.s. 



Fig. 21. — Antero-dorsal part of pharynx in Ascidia- 

 mentula, x 15. br.s. Part of branchial sac ; d.l, 

 dorsal lamina ; d.t, dorsal tubercle ; p.br.z, pre- 

 branchial zone ; p.p, peripharyngeal bands ; sph, 

 sphincter of branchial aperture ; tn, tentacle. 



All the stigmata (of 



which there may be several hundred thousand) in the wall of 

 the branchial sac are bounded by cubical or columnar epithelial 

 cells, which are ciliated. These cilia, so long as the animal is 

 alive, are in constant motion, so as to drive the water onwards, 

 and it is this constant cihary action in the walls of the branchial 

 sac that gives rise to the all-important current of water stream- 

 ing through the body. In addition to the stigmata there are 

 generally one or two much larger elongated slits (Garstang's. 



