CHAPTER Iil 
TUNICATA (CONTINUED) 
CLASSIFICATION: LARVACEA——APPENDICULARIANS—-STRUCTURE, KETC. 
—ASCIDIACEA——-SIMPLE ASCIDIANS—SPECIFIC CHARACTERS 
COMPOUND ASCIDIANS— GEMMATION-— MEROSOMATA— HOLO- 
SOMATA —— PYROSOMATIDAE — THALIACEA — DOLIOLIDAE — 
SALPIDAE——GENERAL CONCLUSIONS——PHYLOGENY. 
WE now turn to the systematic classification of the group; and 
further details of structure or function, points of interest in the 
life-history such as budding and the formation of colonies, the 
habits and occurrence, and other peculiarities such as phos- 
phorescence, will all be noted under the orders, sub-orders, families 
and genera in which they occur. 
CLASS TUNICATA. 
The Tunicata or Urochordata 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 stages. The adult animals are usually 
sessile and degenerate, and may be either solitary or colonial, fixed or 
free. The nervous system is, in the larva, of the elongated, tubular, 
dorsal, Vertebrate type, but in most cases it degenerates 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 or branchial sac, 
which is perforated by two or many more or less modified gill- 
slits opening into a peribranchial or atrial cavity, which communi- 
cates with the exterior by a single dorsal exhalent aperture (rarely 
63 
