EEPTILIA. 



331 



less, as one British species, the Adder (Vipera hems), is poison- 

 ous, it is as well that we should refer very briefly to them. 

 Lizards, however, are decidedly beneficial, for they feed upon 

 noxious insects, &c. Eeptiles are aU provided with a bony 

 skeleton, and may or may not possess legs. The heart is com- 

 posed of four chambers ; but the two 

 ventricles are not completely separate, 

 except in the Crocodiles. The heart 

 (fig. 169) functionally only consists of 

 three chambers, although we have an 

 advance towards the typical completely 

 four-chambered organ. The circulation 

 in Eeptiles is as follows : the impure 

 blood is returned from the body by the 

 large veins (r) and emptied into the right 

 auricle (a) ; from the auricle it passes 

 on to the ventricle {v). The pure arte- 

 rial blood from the lungs enters into 

 the left auricle (a!) and then into the 

 ventricle. Thus the ventricle with its 

 incomplete septum contains mixed blood 

 much as in the frog, this mixed blood 

 being sent by the ventricle to the lungs 

 by the pulmonary artery (_p) and to the 

 body by the aorta (o). We thus get 

 a stage higher than in the Amphibia. 

 Eeptiles, then, like Amphibia, are cold- 

 blooded animals of sluggish habits. They 

 reproduce like Birds, oviparously, the 

 eggs being often laid in strings ; the shells may be hard, but 

 are sometimes soft. All the Crocodilia and most Tortoises 

 lay eggs with a shell just as thick and hard as that of birda 

 Reptile eggs are incubated by the heat of the sun or by the 

 warmth generated by decaying vegetation in which they are 

 often laid. A few cases of an ovoviviparous nature occur, — for 



Fig. 169. — Diagbam op the 

 Circulation in Eeptiles. 



a, Right auricle ; o', left 

 auricle ; v, arterio- venous ven- 

 tricle ; J), pulmonary aitery ; 

 0, aorta. (Nicholson.) 



