75' 



CHAPTER VIII. 



GENERAL CHARACTERS OF ARTICULATED ANIMALS. 



650. THE animals of which is composed the Sub-Kingdom 

 ARTICULATA, not only present an internal structure which is 

 essentially different from that of the other three divisions of the 

 Animal series, hut are likewise distinguished by external charac- 

 ters, which are usually so definite and evident, that they may be 

 almost always recognised at the first glance. Their entire body, 

 in fact, is divided (more or less obviously) into segments ; and 

 seems to he composed of a series of rings placed in a line. In 

 some Articulated animals, this annular (ring-like) appearance 

 results merely from a certain number of transverse folds, which 

 furrow the skin, and encircle the body ; but in the greater num- 

 ber, the animal is enclosed in a sort of solid armour, composed 



FIG. 387- IULUS. 



of a series of rings, united one to another in such a manner as to 

 allow them a certain degree of movement. This envelope has 

 uses analogous to that of the internal framework of Vertebrated 

 animals ; for it determines the general form of the body ; it 

 protects the soft parts; it gives points of attachment to the 

 muscles ; and furnishes them with levers, by their action on 

 which the movements may be effected with precision and 

 rapidity. Hence it may be termed an external skeleton. It 

 must be borne in mind, however, that the hard substance 

 of which it is composed has no analogy in structure, or in 



