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 ahke 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 ds 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. 



