284 



SUBPHYLUM VERTEBRATA: 



general, these changes are related to the transforma- 

 tion from aquatic, plant-eating habits to terrestrial, 

 meat-eating habits. 



CLASS REPTILIA (Reptiles) 



Diagnosis: skin not moist and glandular, but dry 

 and horny, usually covered with scales or plates; 

 usually four legs with five-clawed toes, but legs may 

 be reduced or absent, without paired fins; paired 

 nostrils lead to mouth; respiration by lungs; sexes 

 separate, eggs laid or retained in the female; no larval 

 stage; greatest abundance in warm areas, terrestrial, 

 fresh-water and marine. 



The reptiles include four living orders: the turtles, 

 the snakes and lizards, the crocodilians, and a single 

 species, the tuatara. Their bodies are covered with 

 dry, cornified skin. In many forms, scales or scutes 

 cover the body. They are the first vertebrates so far 

 mentioned that produce a land type of egg. These 

 eggs are large, contain much yolk, and are protected 

 by leathery or calcareous shells. The eggs usually are 

 laid, but some snake and lizard females retain the 

 eggs within their bodies until hatching. 



Unlike amphibians, reptiles can live and be active 

 in dry places, because their more impervious skin 

 retards water loss. In reptiles, the greatest environ- 

 mental restriction is temperature. Although their 

 body heat is obtained from the sun, body temperature 

 is a somewhat complex organismal-environmental re- 

 lationship. Body temperature is not the same as that 

 of the environment. Within limits, an active reptile 

 approaches a constant body temperature, which im- 

 plies some internal temperature regulation. Tem- 

 perature is regulated as tollows: 



When a reptile's body temperature is below opti- 

 mum, the animal basks in the sun. This practice 

 brings its temperature up to the optimum, or normal 

 activity range. Attaining this temperature is expe- 

 dited in some species by their being dark colored 

 when their temperature is below optimum range and 

 lighter colored when it reaches this range. This 

 helps them, because a dark surface absorbs more sun- 

 light and heat than does a light surface. The normal 

 activity range consists of temperatures best suited to 

 life activities such as feeding and reproduction. To 

 maintain this range and not rise to a sernilethal, or 

 even killing, body temperature, the reptiles alternate 

 their activity between cooling shade and warming 

 sunshine. When retreat to shade is insufficient to 



keep the temperature from rising above optimum, a 

 reptile usually goes underground. 



Related to duration of activity is the so-called 

 "third eye" on top of the head and behind the eyes 

 of many species. This structure might be likened to 

 an exposure timing device, because even if an animal 

 can still maintain a temperature within its normal 

 activity range, the animal moves underground after a 

 particular period of exposure. In this regard, if an 

 animal gets insufficient exposure during the earlier 

 part of a day because of extreme temperature, it may 

 come out later in the day when the temperature 

 drops. On the other hand, reptiles may have to go 

 underground or under objects because the environ- 

 ment does not provide sufficient heat for their activity. 

 They then come out when the environment provides 

 sufficient heat to arouse them from their retreat. 



The above emphasis of a normal activity range 

 shows that the cold-blooded, variable-temperatured 

 reptiles show some similarity to the so-called warm- 

 blooded, or constant-temperatured birds and mam- 

 mals. Reptiles' normal activity range, usually within 

 80 to 108° F., is comparable to the temperature range 

 actually displayed by many birds and mammals. The 

 latter, then, are not really much different from rep- 

 tiles. The main difference between the two groups 

 is that reptiles are dependent upon their external 

 environment for a source of body heat and birds and 

 mammals produce their own body heat through 

 chemical activities within their bodies. F"or this 

 reason, reptiles are more accurately called ectotherms 

 (outside temperature) or heliotherms (sun tempera- 

 ture) and birds and mammals, endotherms (inside 

 temperature). One additional difference between 

 ectotherms and endotherms is that healthy reptiles 

 can have either very low or very high body tempera- 

 tures, but both conditions produce lethargy and 

 sometimes death. .An additional consequence of 

 ectothermism is that these animals do not need as 

 much food as endotherms. This gives reptiles an 

 advantage over birds and mammals. However, the 

 endotherms, because their normal body temperature 

 allows immediate activity at all times, can more often 

 react to danger or any other aspect of their environ- 

 ment almost instantaneously. (Further details are 

 given in the discussion of vertebrate dormancy in 

 Chapter 17.) 



The activity of reptiles is not very different from 

 that of am[)hibians. Reptiles display approximately 

 the same annual activity cycle. The main functions 



