MOLLUSCOIDA. 
253 
two directions — towards the Molluscoid^ or higher, and Coelen- 
terate, or lower, divisions of the series. Each of these, in its 
turn, includes a primary and a secondary type, the two secondary 
types, Ascidiozoa and Hydrozoa, being most remote from one 
another and the Ctenophora. The five classes are severally dis- 
tinguished by characters drawn from their alimentary and tenta- 
cular systems. The principal variations in the former are 
ascribed to modification, those in the latter to addition of 
structures. These modifications and additions are either pro- 
gressive or retrogressive, according as they affect the Molluscoida 
or Coelenterata proper. 
TUNICATA. 
The ^ pelagic ^ Tunicata, according to J. D. Macdonald, con- 
stitute a subclass equivalent to the ^ simple,^ social,^ and 
* compound ^ forms of fixed Tunicata collectively, and of greater 
value than either of these groups taken separately. It must 
not be forgotten that the respiratory system, of peculiar import 
in the morphology of this class, appears in the pelagic division 
under four principal modifications, while two, at most*, can be 
recognized among the remaining Tunicata. Again, the pelagic 
genera, though much fewer in number, are sutiiciently diversi- 
fied to represent the three principal sections into which the 
fixed Tunicata are usually divided, as will appear from the an- 
nexed table — 
Tunicata. 
Pelagic. 
Appendicularia ' 
Doliolum . 
Salpa 
Pegea . . . j 
Pyrosoma . 
Fixed. 
. . Simple 
. . Social 
. , Compound 
The author further enlarges on the nature of the distinctions 
involved in this tripartite division of the class, and on the mutual 
relations of the zooicls associated in the same colony among the 
social and compound forms. He adds an analytical table, in 
which all the genera of Tunicata are briefly defined, and arranged 
under groups with special reference to the characters just noticed. 
He has himself established six genera — Per aides, Chondrostachys, 
Diplosoma, Polyclinoides, Pyrosomopsis , and Orthocoela — three of 
* Supposing Chelyosoma and Pelmaia to want the distinct branchial sac 
of the other fixed Ttmicaia. Macdonald here relies on the diagrammatic section 
of -Pclonaia appended by Huxley to his paper in Phil. Trans. 1851. Better 
materials have since led Huxley to conclude that this section is erroneous, 
and that Pelonaia does not differ in any essential respect from Cynthia. The 
case of Chelyosoma is very doubtful. 
