204 



ZOOLOGY. 



Classes. The animals thus arranged under a primary di v i>i i i 

 resemble each other sufficiently to admit of that arrangement, 

 but they differ in many im- 

 portant circumstances, and hence 

 their subdivision into classes. 



379. Thus, amongst verte- 

 brate animals, some are born alive, 

 and are provided with mammae 

 for the nourishment of their 

 young; others spring from an 

 egg, in which they find all the 

 nutriment necessary for their con- 

 stitution, and hence are without 

 organs of lactation ; some respire 

 F,g.i45.-Actima. in the air, others in the water ; 



in some the circulation is complete, in others incomplete; 

 some have the blood hot, in others it is called cold, com- 

 paratively; finally, some are formed to rise into the air, 

 others to live on the ground, and others to swim in the 

 depths of the waters. The differences are of a high physio- 

 logical importance, and coincide, so as to characterize in this 

 division five secondary types ; and hence, to class vertebrate 

 animals according to the principles of the natural method, 

 they must be divided into five classes namely, mammals, 

 birds, reptiles, latrachia, w\& fishes* 



380. In the primary 

 division of the entomozo- 

 aria, or annulated animals, 

 we observe modifications of 

 structure no less remark- 

 able. Sometimes, as in the 

 talitrus, there exist articu- 

 lated limbs serving as levers 

 in the apparatus of loco- 

 motion; and the cephalic 

 portion of the ganglionary 

 nervous system acquires 

 considerable importance. 

 Sometimes, on the contrary, 

 as in the leech, there are no articulated limbs, the nervous 





Fig. 146. Talitrus. 



* In the early editions of this work, the vertebrata were divided into four 

 classes, following Cuvier's arrangement ; the batrachia for good reasons have 

 since been separated from the reptilia. 



