24 



Class II. TUNICATA. 



(ASCIDIANS.) 



The relationship between the animals of the 

 Class just dismissed and those of the present is 

 close and obvious. The cell of the Polyzoan is 

 represented in the Ascidian by the test or tunic, an 

 envelope of cartilaginous, leathery, fleshy, or mem- 

 branous consistence, having two orifices. Within 

 the test is another envelope, distinguished as the 

 mantle, having two orifices corresponding to those 

 of the test. The true digestive stomach is situated 

 at a great distance from the receiving orifice, and 

 near the bottom of the cavity of the body, whence 

 (in the majority of cases) the intestine, bending 

 abruptly upwards, terminates in the secondary ori- 

 fice, a little on one side of the primary (or receiv- 

 ing) one. 



Thus far the parallelism is very close with the 

 POLYZOA. There are, however, important devia- 

 tions from the structure proper to that Class. The 

 crown of radiating tentacles, richly ciliated, is here 

 represented by minute rudiments, the slight impor- 

 tance of which is indicated by their being common 

 to both apertures in some species, and absent from 

 both in others ; while the elaborate array of cilia, 

 which performs so essential a part in the function 

 of respiration, appears under quite another form. 

 The lengthened gullet and crop leading to the 

 stomach in the Polyzoa, is here dilated so as to 

 form a sac of still more enormous dimensions, the 



