18 VERTEBRATE ZOOLOGY 



Flying or parachuting animals, belonging to all of the vertebrate 

 classes except the cyclostomes, exhibit similar parallelisms which con- 

 sist of: first, planes for the support of the weight in the air; and sec- 

 ond, tapering body-form for decreasing the resistance of the body in 

 rapid motion through the air; third, a rigid framework for the planes. 



Arboreal or climbing (scansorial) animals of all vertebrate classes 

 have one prime requisite clinging appendages. Some cling by 

 means of prehensile fingers or tail, others by the use of adhesive pads, 

 others by air-suction pads, and still others by hook-like claws. 



Running (cursorial) animals are very much alike in their mechan- 

 ical adaptations. These are primarily foot modifications. The prime 

 requisite is a long limb with plenty of spring to it. This requisite is 

 met by standing high on the toes with the legs well under the body. 

 Usually there is a progressive loss of some of the digits, a reduction 

 that reaches its climax in the one-toed horses. 



Digging, burrowing (fossorial) animals are usually long and narrow 

 so as to require only a narrow-bore burrow. The fore limbs are 

 strong and have a heavy shoulder girdle for the attachment of the 

 heavy digging muscles. Eyes and ears are usually reduced and the 

 tail is usually poorly developed. 



Desert-dwellers. The prime requisites for these animals are: 

 first, coverings of some sort that prevent undue loss of moisture, 

 such as heavy scales, spines, or armor; second, protection against the 

 extremes of temperature. Many of them meet the latter require- 

 ment by burrowing in sand or in the soil at night. It is also very 

 common for desert creatures to be venomous. This venom may be 

 a chemical end-product of life under arid conditions. 



Cave and deep sea animals. These animals may be viewed 

 largely as products of lowered vitality. It can scarcely be claimed that 

 their characters are on the whole adaptive. One character very com- 

 monly found in abyssmal creatures is phosphorescence. Light-pro- 

 ducing organs of all sorts are developed by these creatures that 

 appear to be definitely adapted to life in the dark. Possibly phos- 

 phorescence may be one of the physiological accompaniments of life 

 under abyssmal conditions. Deep-sea vertebrates (fishes) are also 

 almost invariably forms exhibiting radical distortions of the general- 

 ized fish form. Two principal types are common : those with sup- 

 pressed heads and those with exaggerated heads. This condition is 

 discussed elsewhere. 



