2 VERTEBRATE ANIMALS OF THE UNITED STATES 



the interposition of a flexible body region, which would lessen the driv- 

 ing power of the caudal fin. Fishes, also, in their watery environment, 

 do not depend so much on the sense of sight and the other special 

 senses located in the head to orient themselves as do the land animals, 

 because the density of the medium in which they live limits the carrying 

 power of these senses, but they depend rather upon the lateral line sense 

 and other special senses which reside in the skin over the whole body 

 and are affected directly by the water surrounding it. Fishes are, 

 consequently, often sufficiently oriented with regard to their environ- 

 ment without moving, and may be observed to lie motionless in the 

 water for long periods of time. 



The trunk contains the principal viscera and is the largest and 

 bulkiest body region; in it also are the places of attachment of the 

 two pairs of limbs. In the terrestrial vertebrates the trunk may be 

 subdivided into two or three subregions, the thoracic subregion which 

 with the fore limbs forms the anterior portion, the sacral subregion, 

 forming with the hind limbs the posterior portion, and, in four-footed 

 vertebrates, the lumbar subregion which lies between the other two and 

 forms the pivot where the body bends when the direction of movement 

 is changed. 



The paired limbs are wanting in the cyclostomate fishes, the 

 most primitive vertebrates, and also in snakes and certain species of 

 lizards. The anterior pair is wanting in certain species of fishes, and the 

 posterior pair in many fishes and also in Siren among the amphibians 

 and in the Cetacea and Sirenia among the mammals. The paired 

 appendages of fishes are the pectoral and ventral fins, which are flat- 

 tened swimming organs stiffened by radiating bony and cartilaginous 

 rods. In the other classes of vertebrates they are more or less cylin- 

 drical in form and each is made up of three main divisions. The proxi- 

 mal division projects directly from the trunk and articulates with it; 

 the distal division of the appendages is applied to the substratum or 

 medium on or in which the animal is moving, and is the most complex 

 of the three in structure, terminating typically in the five digits. This 

 number has, however, undergone a reduction in very many vertebrates 

 ■ — 'in extreme cases, like that of the horse, only one digit being present on 

 each foot. In the four-footed vertebrates, also, the fore and hind pairs 

 differ, in most cases, in the number of their digits and the essential 

 uses to which they are put, the hinder pair having a smaller number 

 than the forward pair, as is the case, for instance, in cats and dogs; 

 inasmuch as the hinder pair principally propels the animal, the fewer 

 its points of contact with the ground the greater will be its effectiveness. 



