STRUCTURE OF ASCIDIAN. 



431 



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

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

 (6r), 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 (c?), from which the excrement 

 is removed with the inhaled water 

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

 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 (hr). 

 The latter is in its whole posi- 

 tion and constitution so like 

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

 before anything was known of th-e real relationship of the 

 two animals, the English naturalist, Goodsir, called attention 

 to this striking similarit}'. 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 



