xiv. 5 SKIN OF REPTILES 373 



or lose heat generated within the body. For this reason reptiles are 

 sometimes termed 'ectothermic' and mammals 'endothermic'. These 

 terms are perhaps preferable to 'poikilothermic' (having a variable 

 temperature) and 'homoiothermic' (having a constant temperature) 

 which are in more general use. 



The ectothermic method of temperature control presupposes some 

 sensitive mechanism for registering slight changes in the temperature 

 of the surroundings. There is evidence that the pineal complex is the 

 receptor and that the hypothalamus may be involved in thermal 

 homeostasis. 



It remains true to say, however, that no reptile can retain an inde- 

 pendent body temperature for a long period. For this reason, reptiles 

 living in temperate climes must hibernate during the winter, while in 

 warm countries some, conversely, aestivate during the hottest months. 



4. Skin of reptiles 



The skin is characteristically dry; unlike the skin of amphibians and 

 mammals it contains few or no glands. The Malphigian layer of the 

 epidermis produces the horny scales, which are periodically shed in 

 flakes, or, as in snakes, cast as a single slough. Beneath the horny 

 scales many reptiles (some lizards, crocodiles, some dinosaurs) develop 

 bony plates in the dermis (called osteoderms). These may be re- 

 stricted to the head, where they lie superficial to the skull bones, or 

 may cover most of the body. The tortoise's shell contains both horny 

 (epidermal) and bony (dermal) components (p. 394). The horny 

 scales are often modified to form crests, spines and other appendages. 



Many reptiles, particularly lizards and snakes, have bold and 

 elaborate colour patterns. These may play a part in concealment 

 (though they often seem conspicuous in captive specimens away from 

 their normal terrain). In some forms, especially lizards, there are 

 marked colour differences between the sexes (see p. 407). The well- 

 known phenomenon of colour change, which is much more marked 

 in certain lizards than in any other known reptiles, is discussed on 

 p. 410. 



5. Posture, locomotion, and skeleton 



The elongated body and small laterally projecting legs of many 

 reptiles recall those of a urodele, and the method of locomotion is in 

 general similar in the two groups. Many retain the primitive five digits 

 in both hand and foot. With the similarity of movement goes a general 

 similarity in plan of the skeleton: there are, however, certain most 



