74 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
APPENDIX. 
It may be useful to add here a brief statement of the 
classification and characters of the TUNICATA, in order to 
indicate the position of Ascidia as a type of the group, 
and its relations to the other British Ascidians.* 
TUNICATA. 
The Tunicata (or Urochorda) are hermaphrodite marine 
chordate animals, which show in their development the 
essential vertebrate characters, but in which the notochord 
is restricted to the posterior part of the body, and is in 
most cases present only during the free-swimming larval 
stage. The adult animals are usually sessile and degener- 
ate, and may be either solitary or colonies, fixed or free. 
The nervous system is in the larva of the elongated, 
tubular, dorsal vertebrate type, but in most cases degener- 
ates in the adult to form a small ganglion placed above 
the pharynx. The body is completely covered with a thick 
cuticular test (‘‘tunic’’), which contains a substance similar 
to cellulose. The alimentary canal has a greatly enlarged 
respiratory pharynx (the branchial sac), which is perforated 
by two, or many, more or less modified gill slits, opening 
into a peribranchial or atrial cavity, which communicates 
with the exterior by a single dorsal exhalent aperture, 
rarely two ventral apertures. The ventral heart is simple 
and tubular, and periodically reverses the direction of the 
blood current. 
* For a more detailed classification, with definitions of all the groups and 
analytical keys to the species, see Herdman’s Revised Classification of the 
Tunicata. Journ. Linn. Soc.—Zool., vol. XXIII., p. 558, 1891. 
