142 BRITISH TUNICATA. 



tentacular filaments (PL XL VII, fig. 4) are small, 

 linear, and simple, nearly of equal size, from twelve to 

 fourteen in number, arranged a little above the anterior 

 margin of the branchial sac. 



The branchial suspenders are well developed, and, 

 as they are attached to the walls of the primary vessels, 

 when the animal is contracted they are liable to draw 

 the branchial organ into transverse wrinkles ; this 

 constantly happens with all those forms which have 

 that organ much elongated. 



The heart (PL XL VII, fig. 1) is a long, rather 

 narrow, tube ; it lies between the mantle and lining 

 membrane on the left side, in a by no means wide 

 pericardial chamber. The dorsal end advances a short 

 way upwards, and on reaching the endostyle there 

 terminates ; the ventral extremity abuts against the 

 cardiac end of the stomach, and sends a large trunk 

 vessel to either side of that organ. The systemic 

 circulation has not been further traced ; the branchial 

 circulation, however, is evident enough, and differs in 

 no respect from that in Ascidia. 



The nerve-ganglion (PL XLVII, fig. 4; PL XL VIII, 

 fig. 18) is rather bulky, being less attenuated than usual, 

 and the branchial tubercle rests upon its anterior ex- 

 tremity. 



The mouth (PL XLVII, figs. 1-3) is circular, and 

 opens into the base of the branchial sac towards the 

 ventral margin. The oesophagus (PL XLVII, figs. 

 13) is much longer than usual, and passes downwards 

 by the ventral margin almost to the bottom of the 

 pallial chamber. It then suddenly turns upwards to 

 reach the lower end of the stomach, which is an elon- 

 gated, pyriform organ placed diagonally below the 

 branchial sac, the attenuated or pyloric extremity 

 passing upwards in the direction of the dorsal margin 

 of the chamber. The intestine (PL XLVII, figs. 1-3) 

 leaves this extremity of the stomach, and, ascending a 

 little, turns inwards and downwards, and forms a small 

 narrow loop. It then crosses to the ventral margin of 



