26 INTRODUCTION. 



verse folds into a certain number of rings, whose teguments are some- 

 times soft, and sometimes hard ; the muscles, however, being always 

 situated internally. Articulated limbs are frequently attached to the 

 trunk ; but very often there are none. We will call these animals 



AM MAMA ARTICULATA, 



Or articulated animals, in which is observed the transition from 

 circulation in closed vessels to nutrition by imbibition, and the corre- 

 sponding transition from respiration in circumscribed organs to that 

 effected by tracheae or air-vessels distributed throughout the body. In 

 them, the organs of taste and sight are the most distinct ; one single 

 family alone presenting that of hearing. Their jaws, when they have 

 any, are always lateral. 



The fourth form, which embraces all those animals known by the 

 name of Zoophytes, may also properly be denominated 



AMMALIA RADIATA, 



Or radiated animals. We have seen that the organs of sense and 

 motion in all the preceding ones are symmetrically arranged on the 

 two sides of an axis. There is a posterior and anterior dissimilar face. 

 In this last division, they are disposed like rays round a centre ; and 

 this is the case even when they consist of but two series, for then the 

 two faces are similar. They approximate to the homogeneity of plants, 

 having no very distinct nervous system or particular organs of sense ; 

 in some of them, it is even difficult to discover a vestige of circulation ; 

 their respiratory organs are almost universally seated on the surface 

 of the body, and the lowest of the series are nothing but a sort of 

 homogeneous pulp endowed with motion and sensibility. 



