354 
THE GEOLOGIST. 
the leech, and in the various modifications offered in the lobster and 
crab-tribes, the spiders, the centipedes, and ordinary insects, as 
beetles, moths, and flies. 
All these classes (termed, as a whole, Invertebrates) are remark- 
able for the absence of any infernal skeleton or support, except in 
the cuttle-fish, the "pen" or solid plate of which is internal, although 
not approaching the character of a framework of bones, while the 
animal is otherwise, through the nautilus thoroughly identified 
with the more usual characters of the mollusca. 
Between this class and the time Mollusca are certain animals 
which possess more or less similar parts and organs to the shell- 
fish proper, but which yet differ fi-om them in either being seated 
in cup-Uke cells linked together by a horny or calcareous stem, 
or in being enveloped in a bag-like skin or tunic, and in being 
divested of any proper shell. These are termed MoUuscoidea, or 
Mollusca- (shell-fish) resembling animals. Such are the Bryozoa 
or moss-like encrusting- animals — Flustra, Eschara, Plumatella, 
&c., and both the simple and the compoimd Ascidians or 
Tunicata. 
In the Mollusca (soft-bodied animals) or shell-fish are included 
very considerable ranges of development in the rank of organic Ufe. 
All possess a stomach, nerves in the form of ganglionic cores and 
threads, organs of digestion, and more or less of locomotion, and a 
single (univalve) or double (bivalve) sohd calcareous shell. Some, 
the lowest in grade, are headless, as the oyster ; others are of high 
organization, and approach towards the fishes, namely, the Cepha- 
lopoda, or cuttle-fish. 
The next great division of the animal-kingdom, which we now 
approach, is characterized on the contrary by the possession of an 
internal framework of osseous supports or bones, and especially of a 
continuous series of a particular form, vertebra, connected together 
by muscles and ligaments into a vertebral column or back-bone, as 
it is familiarly termed. This vertebral column is cartilaginous 
and almost rudimentary in some fishes, while in others it is com- 
paratively solid and bony. In the Fish-tribe the whole column is 
adapted to flexible motion, generally lateral or from side to side, 
and the ribs, skull, and fins are also modified for the free propul- 
