CHAPTER XVIII 



PHYLUM CHORDATA 

 SUB-PHYLUM 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 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. Nevertheless 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 sedentary, tend to diverge very widely from the 

 Vertebrate type. Thus the nervous system is generally re- 

 duced to a single gatiglion placed above the pharynx. The 



499 



