ASCIDIA. 75 
The leading vertebrate characteristics of the Tunicata 
are the notochord, the dorsal nervous system, the ventral 
heart, and the respiratory pharynx with gill clefts, but 
these all disappear or undergo modification to such an 
extent, during the metamorphosis, that the degenerate 
adults would not, in most cases, be recognised as belong- 
ing to the chordata were it not for our knowledge of the 
life-history. 
The class Tunicata may be divided into three orders :— 
Order I. LARVACEHA. 
This comprises the free-swimming, permanently-tailed, 
larva-like, mostly minute Appendicularians. A relatively 
large test or ‘‘ House”’ 1s formed with great rapidity as a 
secretion from the surface of a special part of the ecto- 
derm ; it is, however, merely a temporary structure, which 
may be cast off and afterwards replaced by another 
““House.”’ The branchial sac is simply an enlarged pharynx, 
with two ventral ciliated openings (stigmata) leading to 
the exterior. These open independently on the ventral 
surface, and there is no separate peribranchial cavity. 
The tail is a large locomotor appendage, in which there 
is a skeletal axis, the urochord, comparable with the noto- 
chord of Vertebrata. The nervous system consists of a 
large anterior and dorsally-placed ganglion, and a long 
nerve cord with smaller ganglia stretching backwards 
from it over the alimentary canal to reach the tail, along 
which it runs on the left side of the urochord. The 
alimentary canal lies behind the branchial sac, and the 
anus opens ventrally on the surface of the body in front 
of the stigmata (or atriopores). The gonads are placed at 
the posterior end of the body. Gemmation does not take 
place, and alternation of generations and metamorphosis 
do not occur in the life-history. 
