ANATOMY AND PHYSIOLOGY. 



43 



lower rows, and downwards for the two upper rows/' If tin- 

 heart in the first instance threw the blood into, instead of 

 drawing it from, the gill, the reversal of this motion would 

 take place; namely, the flow in the secondary vessels above the 

 point indicated would be upwards, and downwards below it. 



Such are the characters of the branchial sac as seen to 

 exist in A. venosn. The minute network, however, is not 

 continuous throughout the whole organ, but is interrupted in 

 such a manner as to show that it is composed of two lateral 

 lobes or lamina?. It is divided along the dorsal line by two 

 parallel folds of the lining membrane, which are separated 

 by a deep groove; the tissue at the base of each fold is 

 stiffened by a flattened rod of a somewhat rigid, opaque, 

 yellowish substance, which together form the endostyle, that 

 lies, as it were, in the bottom of the groove, along which the 



FIG. 5. 



FIG. 6. 



FIGS. 5 AND 6. Transverse sections of the endostyle of Ascidia mentula, 

 highly magnified. The shaded portion represents the dorsal blood-sinus, 

 the thick black line the endoderm. Fig. 5, before separation of the lips. 

 Fig. (3, after separation. 



rods appear to be united (see PL III, fig. 5, and figs. 5 and 6 

 in text). The upper extremities of these folds diverge right 

 and left, and become continuous with the lower member of 

 what may be termed the anterior cord or collar two narrow 

 folds also of the lining membrane that encircle the base of 

 the respiratory tube, a little above the anterior margin of the 

 branchial sac, and having the circular blood-channel, pre- 

 viously mentioned, immediately below them. The ventral 

 margin of the sac is furnished with a wide, longitudinal, 

 delicate, membranous fold, which apparently also orginates 

 in the lining membrane, and which interrupts the continuity 

 of the minute network in this direction. This is the ventral 

 or oral lamina (PI. XIX) ; it extends from end to end of the 

 branchial sac, and is ribbed transversely; the margin is entire. 

 The mouth opens close by its left side, about one-third from 

 the lower extremity : the upper extremity for some little way 

 downwards is divided longitudinally, showing that the lamina 



