CHAPTER XVIII. 



PHYLUM CHORDATA. 



SUB-PHYLUM UROCHORDA OR TUN 1C AT A. 

 (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 the development of a simple Ascidian, and 

 correlated it, step by step, with that of Amphioxus. He 

 showed that the larval Ascidian has 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 along with the Chordates. 

 Of their possible relations to simpler forms nothing definite 

 is known. 



GENERAL CHARACTERS. 



The Tunicates are marine Chordata, but the chordate 

 characteristics --dorsal tubular nervous system, notochord, 

 gill-slits, and brain eye are in most cases discernible only 

 in the free-swimming larval stages. They usually degenerate 

 in the course of their development, and the adults, which 

 are in most cases sedentaiy, tend to dive?'ge very widely 

 from the Vertebrate type. Thus /he nervous system is 

 generally reduced to a single ganglion placed above the 

 pharynx. The body is invested by a thickened articular 



