PEIHARY DIVISIONS AND CLASSES, 



215 



resemble each other sufficiently to admit of that arrangement, 

 but they differ in many important circumstances, and hence 

 their subdivision into classes. 



379. Thus, amongst vertebrate animals, some are born 

 alive, and are provided with mammae for the nourishment of 



Fig. 170. lulus ; Millipede. 



their 3 T oung ; others spring from an egg, in which they find 

 all the nutriment necessary for their constitution, and hence 

 are without organs of lactation ; some respire 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 . 



Fig. 171. Agrion ; Dragonfly. 



\ 



Fig. 172. Bethylus. 



called cold, comparatively : 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 

 physiological importance, and coincide, so as to characterize 

 in this division five secondary types ; and hence, to class 



strange animals (amorphozoaria) present when young the same characters as 

 polyps, only their organic development is arrested at a transitory state, and 

 they become deformed as they grow older. Thus by keeping in view their 

 mode of development, they may be referred to the class zoophytes. 



