CHAPTER XVIII 
PHYLUM CHORDATA 
SUB-PHYLUM UROCHORDA or TUNICATA 
(Ascipians, SEA-SQUIRTS, ETC.) 
Tue 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 /arval 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 ave 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 1s generally re- 
duced to a single ganglion placed above the pharynx. The 
