112 ANIMAL BIOLOGY 



essentially as parasites within the body of the female until birth 

 occurs. (See: Appendix I.) 



It will be appreciated, of course, that other important charac- 

 teristics — many of which will be apparent as we proceed — 

 delineate these chief Vertebrate groups; but, in fact, the Verte- 

 brates as a whole are remarkably homogeneous both structurally 

 and functionally, the most obvious external differences to the 

 contrary. Some of the outstanding characters typical of Vertebrates, 

 in addition to the unique notochord, living endoskeleton, dorsal 

 nerve tube (spinal cord), and gill slits, are bilateral symmetry, 

 traces of segmentation, coelom, red blood corpuscles, brain encased 

 in a skull, paired appendages (fins or limbs), and a tail. (Fig. 94.) 



Vertebrates are the modern animals: the "athletes of the Animal 

 Kingdom." They have parcelled out, as it were, the available 

 environment amongst themselves. The Fishes dominate the waters, 

 the Birds, the air, and the Mammals, the land. To be sure, the 

 Amphibians waver between water and land, and the Reptiles are 

 chiefly terrestrial; but both are minor groups to-day: the suprem- 

 acy of the Reptiles passed to the Mammals in the geological yes- 

 terday. Man is a Mammal. 



A. Fishes 



Living as they do in an aquatic environment, Fishes find at 

 least two problems of large-bodied, active, terrestrial animals con- 

 siderably simplified. In the first place, the density of water makes 

 less necessary either supports to raise the body or sturdy muscles 

 to move them. Thus the paired appendages, fins, and the tail of 

 Fishes are adapted solely for propulsion and steering. In the 

 second place, although an efficient respiratory apparatus is re- 

 quired, no special provision is needed to maintain the respiratory 

 membranes moist. The water merely passes into the mouth, over 

 the gills, and then out through the gill slits. (Fig. 117.) 



Fishes are cold-blooded since they possess no mechanism to 

 maintain a constant body temperature — a character they share 

 with the Amphibians and Reptiles. Most species are oviparous — 

 the eggs are shed; but some, such as the well-known Guppy, are 

 viviparous — the eggs develop within the mother's body and the 

 young are born. 



If we neglect the primitive fish-like creatures devoid of true jaws 

 and paired fins, known as Cyclostomes, and the peculiar 



