CHAPTER XVIII. 



Class UROCHORDA or TUNICATA. 



(Ascidians, Sea-Squirts, etc.) 



The Tunicates are remarkable animals, which seem to 

 stumble on the border line between Invertebrates and 

 Vertebrates. They were classified with Polyzoa and 

 Brachiopoda as Molluscoidea, until, in 1866, Kowalevsky 

 described for the first time the development of a simple 

 Ascidian, and correlated it, step by step, with that of 

 Amphioxus. He showed that the larval Ascidian possesses 

 a dorsal nerve-cord, a notochord in the tail region, gill-slits 

 opening from the pharynx to the exterior, and an eye 

 developing from the brain. It is true that in most cases 

 the promise of youth is unfulfilled ; the active larva settles 

 down to a sedentary life, loses tail and notochord, nerve- 

 cord and eye, and becomes strangely deformed. Neverthe- 

 less we must now class Tunicates as degenerate Vertebrates. 

 Of their possible relations to simpler forms nothing definite 

 is known. 



General Characters. 



The Tunicates are marine Chordata, but the chordate 

 characteristics — dorsal nervous system, notochord, gill-slits, 

 and brain eye — are in most cases discernible only in the free- 

 swimming larval stages. They usually degenerate in adoles- 

 cence, and the adults, which are in most cases sedentary, tend 

 to diverge very widely from the Ve?-tebrate type. Thus the 

 nervous system is generally reduced to a single ganglion. The 

 body is invested by a thickened cuticular test, which contains 



