STRUCTURE OF ASOIDIAN. 



431 



able cavity, filled with water; this is the gill-cavity , or 

 respiratory cavity (Fig. 153, cl; Plate XI. Fig. 14, cl). It 

 is also called the mantle or tunic cavity, because it receives, 



Fig. 153. — Structure of an Ascidian 

 (viewed from the left side, as in Plate 

 XII. Fig. 14) ; the dorsal side is turned 

 towards the right, the ventral side to- 

 wards the left, the mouth-opening (o) 

 upwards ; at the opposite, tail extremity, 

 the ascidian is firmly attached to some 

 substance below. The gill-intestine 

 (hr), which is pierced by many open- 

 ings, continues below as the stomach- 

 intestine. The large intestine opens 

 through the anus (a) into the gill- 

 cavity (cl), from which the excrement 

 is removed with the inhaled water 

 through the mouth of the tunic (a') ; m, 

 tunic. (After Gegenbaur.) 



not only the water for respir- 

 atory purposes, but also ex- 

 crement and the sexual pro- 

 ducts. The greater part of the 

 respiratory cavity is occupied 

 by the latticed gill-sac (br). 

 The latter is in its whole posir 

 tion and constitution so like 

 the gill-body of the Amphioxus, that many years ago, 

 before anything was known of the real relationship of the 

 two animals, the English naturalist, Goodsir, called attention 

 to this striking similarity. In the Sea-squirts also the 

 mouth -opening (0) leads directly into this gill-sac. The 

 water breathed in passes through the openings of the 



