INVERTEBRATA. 451 



requires a special organ of respiration, which is supplied 

 by the allantois, and the embryo is also enclosed in the 

 amnion. These three classes are therefore grouped 

 together as the Amniota, contrasted with the Fishes and 

 Amphibia which form the Anamniota. Reptiles are cold- 

 blooded, that is to say the temperature of the blood is not 

 much above that of the surrounding medium, and varies 

 with that of the latter, while in birds and mammals it is 

 not only high but constant. In Eeptiles two aortic arches 

 are present, as in the frog, and there are two auricles and 

 one ventricle, except in Crocodilia, in which the ventricle is 

 completely divided. The skin is almost entirely destitute 

 of glands, and the outer layers of the epidermis are thick- 

 ened and cornified in distinct areas, either meeting edge to 

 edge (scutes) or overlapping (scales). The scales of Eeptiles 

 are therefore very different from those of fishes, which are 

 dermal structures ; many Eeptiles, however, have also 

 dermal ossifications, for example, the crocodiles and turtles 

 and many lizards, but these do not in all cases correspond 

 to the epidermic scales. The skull articulates with the 

 vertebral column by a single condyle, and has more 

 cartilage bones than in the Amphibia ; some of the anterior 

 vertebrae are included in the skull, so that the hypoglossal 

 is a cranial nerve. The conus arteriosus is merged in the 

 heart. The metanephros is entirely separated from the 

 mesonephros, the latter forming the epididymis in the male 

 and being vestigial in the female ; this is true also of all 

 the Amniota. In Eeptiles there is a renal portal system 

 of veins. There are four orders of living Eeptiles : the 

 Rhyncocephalia, containing only a single genus, Hatteria, 

 confined to New Zealand, which has many primitive 

 features ; the Squamata, in which the quadrate bone is 

 moveable, and which include the lizards and snakes ; the 

 Chelonia, or tortoises and turtles, distinguished by the com- 

 plete bony investment formed by the fusion of dermal bony 

 plates with the endoskeleton ; and the Crocodilia, which 

 have bony dermal plates covered with sculptured horny 

 plates of corresponding shape on the dorsal surface, and in 

 some cases on the ventral surface also; the ventricle of 

 the heart is completely divided, but the oxygenated and 



