222 



TUNICATA, OR ASC1DIANS. 



PAET III. 



SECTION X. 



TUNICATA, OR ASCIPIANS. 



THE form of the Tunicata is irregular. They have 

 two orifices one at the top, for the entrance of a cur- 

 rent of water, and another at one side for its egress. 

 They have two tunics only adhering to one another at 

 the edges of these orifices, which are furnished with a 

 circle of cilia. The irregular or scattered condition of 

 the nerve-centres, as well as the alternation in the cir- 

 culation, are eminently characteristic of the whole class. 

 They consist of three distinct groups : the compound 

 or social gelatinous Ascidians ; the solitary Tunicata, 

 with leathery coats ; and the Salpse, 

 which are gelatinous. The two 

 first, though mobile when young, 

 become fixed when they arrive at 

 maturity ; the third floats free on 

 the surface of the ocean. 



The fixed gelatinous Ascidians 

 resemble the Polyzoa in structure 

 and tendency to gemmation; never- 

 theless, they differ in their circu- 

 lating and respiratory systems. The 

 Perophora Listeri is an example 

 which is found on the south coast Fi e- 16 - 

 of England and Ireland (fig. 160.) 

 It consists of minute globes of clear jelly, not larger 

 than a pin's head, spotted with orange and brown, 



group of 



