354 THE GEOLOGIST. 



the leech, and in tlie various modifications offered in tlie lobster and 

 crab-tribes, tlie spiders, tlie centipedes, and ordinary insects, as 

 beetles, motbs, and flies. 



All these classes (termed, as a whole. Invertebrates) are remark- 

 able for the absence of any internal 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, tlu'ough the nautilus thoroughly identified 

 with the more usual characters of the mollusca. 



Between this class and the true ^Mollusca are certain animals 

 which possess more or less similar parts and organs to the shell- 

 fish proper, but which yet differ from them in either being seated 

 in cup-hke 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 Molluscoidea, or 

 Mollusca- (shell-fish) resembling animals. Such are the Bryozoa 

 or moss-like encrusting- animals — Flustra, Eschara, Pluniatella, 

 &c., and both the simple and the compound Ascidians or 

 Tunicata. 



In the Mollusca (soft-bodied animals) or shell-fish are included 

 very considerable ranges of development in the rank of organic life. 

 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) solid 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- 



