ANATOMY AND DEVELOPMENT. 187 



is free, so that the right and left halves of the atrial 

 cavity are continuous ventrally, instead of dorsally. 



In order to see the stomach and intestine, it is necessary 

 to cut through the left wall of the pharynx, since the vis- 

 cera lie, at least in the genus Ascidia (or Phallusia), on 

 the left side of the pharynx. It should be pointed out 

 that the topographical arrangements vary considerably 

 among the different genera of Tunicates. In Clavelina, 

 for example, the viscera lie behind the pharynx, as shown 

 in Fig. 96. 



On the left side of the pharynx (Fig. 94) the short 

 oesophagus leads into the dilated stomach, which again 

 narrows down to the looped intestine, and finally the lat- 

 ter bends sharply forwards into the rectum, which opens 

 by the anus into the atrial cavity, the excrement being 

 carried to the exterior by the constant stream of water 

 which flows out through the atrial or cloacal aperture. 



Instead of being straight, as in Amphioxus, the aliment- 

 ary canal is here doubled round upon itself. This U-shaped 

 character of the alimentary canal of Ascidians is shown 

 with great clearness in the case of Clavelina (Fig. 96), 

 where there are no secondary convolutions in the course 

 of the intestine. 



The Ascidians are one and all hennap/irodite, and the 

 reproductive glands frequently lie between the loops of 

 the intestine, while two ducts, oviduct and vas deferens, 

 which often present the appearance of a single duct with 

 a double lumen, proceed forwards by the side of the rec- 

 tum, to open into the cloacal region of the atrial cavity 

 near the anus (Fig. 94,^ and gd). 



The ovary and testis, though quite separate in the adult, 

 originate, according to the account given by the Belgian 

 zoologists, Edouard van Beneden and Charles Julin, 



