48 



ASCIDIANS 



pharyngo-cloacal slits) placed close to the dorsal lamina and 

 leading direct to the cloaca. 



rig. 22 shows a small part of the wall of the branchial sac, in 

 which it may be seen that the bars containing the blood- 

 channels are arranged in three regular series: — (1) The "trans- 

 verse vessels " which run horizontally round the wall and open 

 at their dorsal and ventral ends into large median longitudinally 

 running tubes, the dorsal blood-sinus (or " dorsal aorta ") behind 

 the dorsal lamina, and the ventral blood-sinus (or " branchial 

 aorta ") beneath the endostyle ; (2) the fine longitudinal or 



" interstigmatic vessels " which 

 run vertically between adjacent 

 transverse vessels and open into 

 them, and which therefore bound 

 the stigmata ; and (3) the " in- 

 ternal longitudinal bars " which 

 run vertically, in a plane internal 

 to that of the transverse and 

 fine longitudinal vessels. These 

 bars (Fig. 22, i.l) communicate 

 with the transverse vessels by 

 short side branches where they 

 cross, and at these points are 

 prolonged into the cavity of 

 the sac in the form of hollow 

 papillae. In some Ascidians 

 (e.g. Corella and most of the 

 Molgulidae) the interstigmatic 

 vessels are curved so that the stigmata form more or less com- 

 plete spirals (see Figs. 35 and 41). In some species of Ascidia, 

 and other Ascidians, the interstigmatic vessels are inserted into 

 the transverse vessel in an undulating course in place of the 

 straight line seen in Fig. 22, B, l.v, the result being that the 

 stigmatic part of the wall of the branchial sac seems to be folded 

 or thrown into microscopic crests and troughs. This is known as 

 " minute plication." In some cases, again (Cynthiidae), the 

 whole wall of the sac is pushed inwards at intervals to form 

 large folds visible to the eye (see Fig. 36, A and B). The 

 intersections of the internal longitudinal bars with the trans- 

 verse vessels divide up the inner surface of the branchial 



Fig. 22. — A mesh of the hrancliial sac of 

 Ascidia, seen A, from inside ; B, in 

 horizontal section. c.d, Connecting 

 duct ; h.in, horizontal membrane ; i.l, 

 internal longitudinal hars ; Lv, inter- 

 stigmatic vessels ; p, p', papillae ; sg, 

 stigmata ; tr, transverse vessel.s. 



