//^■^ '^''"'■■^■^ 



rhinariunr 



OPOSSUM 



Figure 2-12. The marsupial and placental mammals. 



a part of their early development. The nipples of the mam- 

 mary glands are in this pouch, which gives the group its 

 name. The eyes are large and prominent and there are 

 large, bare auricles (ears) (Figure 2-12). The body is covered 

 with long hair. The rat-like tail, covered with scales, is pre- 

 hensile, being used by the animal to hang from branches. 



Placenta/ The body of the rat (Figure 2-12) is covered 

 with dense hair, and its long tail is scaled, with some hairs 

 growing out between the scales. The limbs are similar to the 

 opossum's. There is no marsupium; the nipples are arranged 

 in bilateral lines along the belly. The young remain within 



the mother's body for a longer developmental (gestation) 

 period. They are nourished through a special structure, the 

 placenta. 



FOSSIL VERTEBRATES 



An examination only of living forms results in overlook- 

 ing many interesting groups represented by fossils. Some 

 of these will be introduced here, whereas others will be re- 

 ferred to in the discussion of specific anatomical areas. 



Well-known fossil agnath fishes, related to the cyclo- 

 stomes, include such genera as Tremalaspis, Cephalaspis, 



ronchial opening 



LASANIUS 



Figure 2-13. Several kinds of fossil agnaths as they are presumed to have looked. 



30 • THE VERTEBRATES AND THEIR EXTERNAL ANATOMY 



