68 BRITISH TUNICATA. 



lobe, and which also exhibit a tendency to dichotomous 

 division ; these are the male secreting vesicles. 



In A. scaLra, A. affinis, A. niri/tnlx, A. venosa, etc., 

 the same arrangement of the reproductive organs 

 obtains, but the ovary in A. iiit.'iitiiJn is lobulated, and, 

 lying within the loop of the intestine, is seen at both 

 sides of the alimentary tube, and consequently has the 

 appearance of being double. In A. renosa the male 

 vesicles are exceedingly minute and very numerous. 



The heart in Asciditi is tubular and is attached to 

 the lower or posterior border of the stomach, one end 

 extending some way up the dorsal region towards the 

 intestinal tube ; this is the dorsal extremity ; the other, 

 the ventral extremity, points in the direction of the 

 oesophagus. It lies between the mantle and the lining 

 membrane within a distinct chamber or pericardium, 

 along one side of which it is attached from end to end 

 (Plate III, fig. 4). 



A large trunk vessel passes from the dorsal extremity 

 of the heart and immediately divides into three branches, 



u 



one of which advances along the dorsal region at the 

 back of the endostyle ; another passes in the opposite 

 direction down the dorsal margin to the bottom of the 

 branchial sac. These two form the dorsal branchial 

 channel, and are equivalent to the ventral or thoracic 

 sinus of Milne-Edwards ; and they both communicate 

 with the dorsal extremities of the transverse channels 

 or primary vessels of the branchial sac. The third 

 branch turns off at right angles to this great dorsal 

 channel close to the point where it is united to the 

 heart, and, in company with another vessel to be 

 shortly described, penetrates the mantle, and goes to 

 ramify in the test. 



From the other extremity of the heart there are two 

 lars'e trunk vessels siven off, one to each side of the 



O o * 



stomach. These ramify over the digestive organs and 

 supply the pallial plexus. The trunk branch of the 

 left side divides into two large stems, one of which 

 inclines towards the intestine, the other towards the 



