386 ZOOLOG Y. 
Cuiass I.—Tounicata (Ascidians, Sea Squirts). 
General Characters of Tunicates.—These animals were 
once regarded as mollusks, and in former editions of this book 
they were assigned a position among the worms, between the 
Brachiopods and the Nemertina. 
Recent advances in our knowledge of Ascidians on the 
one hand, and of the primitive features of the Vertebrates on 
the other, show quite conclusively that the Ascidians, par- 
ticularly the adult form Appendicularia, and the larve of 
those Ascidians which undergo a metamorphosis, have the 
fundamental characters of Amphioxus and the embryos of 
genuine Vertebrates, such as the lamprey. 
It will be remembered that these fundamental characters 
are the presence of a notocord, over which lies the central 
nervous system. No invertebrate is known to possess 
this dorsal position of the nervous system to the dorsal cord, 
unless we except Balanoglossus, which, as Mr. Bateson has 
shown, has a notocord lying under a central nervous cord. 
If the larva of this form was not like that of the worms and 
Echinoderms, presenting no vertebrate features, we might 
adopt Bateson’s view that Balanoglossus should be placed 
at or near the base of the Vertebrate series, in a group Pro- 
tochordata. 
The result of admitting the Tunicates into the same 
branch or type as the Vertebrates has led to the proposal of 
a group Chordata, including the Tunicates and the genuine 
Vertebrates; but as Amphioxus seems to be a connecting- 
link between the Tunicates and the genuine Vertebrates, 
beginning with the hag-fish and the lamprey, we will, for 
convenience, retain the familiar word Vertebrata for all ani- 
mals having a notocord (either in the embryo, larval, or 
adult state) situated between a neural and an enteric cavity. 
Fig. 386° will show the close resemblance of the larval as- 
cidian to the embryo lamprey. 
It will be seen that even the larval Ascidian has an incipi- 
ent brain, consisting of two ganglia, from which arise a 
spinal nervous cord, with even spinal nerves. The intestine 
in the larval Ascidian is bent and ends in front, but in the 
adult tadpole-shaped Appendicularia the end of the intes- 
tine is ventral and opens directly outwards. 
