CHAPTER XVIII. 



CLASS UROCHORDA OR TUNICATA. 

 (AsciDiANS, Sea Squirts, &c.) 



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 

 Ainphioxns. 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 mari)ie 

 Chordaia, but the chordate characteristics — dorsal ?iervous 

 system, notochord, gill slits, and brain eye — are in most cases 

 discernible only in the free swimming larval stages. They 

 usually degenerate in adolescence, and the adults, which are in 

 fnost cases sedentary, tend to diverge very 'widely from the 

 Vertebrate type. Thus the nervous system is generally reduced 

 to a single ganglion. The body is invested by a thickened 

 cuticular tunic, which contains cellulose. The pharynx is per- 

 forated by two (Larvacea), or in the majority by numerous 

 ciliated gill slits, and is surrounded to a greater or less extent 



