16 



back along the right side of the dorsal lamina to the 

 ccsophagus. 



The dorsal lamina is at its widest round the oesophageal 

 aperture (PL 11., fig. 4, oes.). It is more or less ridged 

 transversely, especially on its convex left side, and may 

 have marginal tags or teeth (PI. IV., fig. 4), which in 

 some Ascidians become long processes, the langnets. 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. 



Wall of Branchial Sac. 



Fig. 3, PI. II. shows a small part of the wall of the 

 branchial sac of Ascidia in which it may be seen that the 

 bars containing the blood channels are arranged in three 

 regular series : — 



(1) The "transverse vessels" {tr.) which run horizon- 

 tally round the wall and open at their dorsal and ventral 

 ends into large median longitudinally running tubes, the 

 dorsal blood sinus behind the dorsal lamina and the ventral 

 blood sinus below the endostyle (see also PL III., fig. 10). 



(2) The fine longitudinal or " interstigmatic vessels " 

 {l.v.) which run vertically between adjacent transverse 

 vessels and open into them, and which therefore bound 

 the stigmata. 



(3) The "internal longitudinal bars" {i.l.) which run 

 vertically in a plane internal to that of the transverse and 

 fine longitudinal vessels. These bars (see PL II., fig. 3, B.) 

 communicate with the transverse vessels by short side 

 branches {c.d.) where they cross, and at these points are 

 prolonged into the cavity of the sac in the form of hollow 

 papilkie {p.). 



In sonie Ascidians {e.g. Corella parallelog ramma and 

 most of the Molgulidae) the interstigmatic vessels are curved 



