CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 343 



in a cloaca or opens by an anus in front of the urinogenital 

 apertures. Blood at about same temperature as surrounding 

 medium, and containing large oval nucleated red corpuscles. 

 Heart two- or three-chambered, and aortic arches never less 

 than two. Gills, and at least four pairs of branchial arches, 

 always present at some period of life. The coelom is not 

 divided into thoracic and abdominal sections by a respiratory 

 diaphragm. The meson ephros persists. The cerebral hemi- 

 spheres are never united by a conspicuous corpus callosum, 

 the hypoglossal nerve does not perforate the brain-case, and 

 the sclerotic coat of eye, when present, is cartilaginous or 

 bony. 



Class 1. Pisces (Scyllium). Ichthyopsida with fin-like 

 limbs, the margins of which are supported by fin-rays, 

 as also are the unpaired fins. A well-developed exo- 

 skeleton mainly of dermal origin. No postcaval vein 

 and no allantois (see p. 153). 



Class 2. Amphibia (Rand). Ichthyopsida in which 

 the unpaired fins always present in the larva and some- 

 times in the adult are not supported by fin-rays. The 

 limbs are transversely jointed and their extremities 

 divided into digits. There is no epidermic exoskeleton, 

 and the skin is very glandular. A cloaca and an allan- 

 toid bladder. 



B. A m n i o t a. Embryo with amnion and respiratory allantois. 

 Gills never present. 



GROUP 2. SA UR OP SID A. Air-breathing Amniota, 

 with epidermic exoskeleton of scales or feathers. The ver- 

 tebrae are without epiphyses, the cranium is well ossified 

 and articulates with the vertebral column by a single occi- 

 pital condyle, partly belonging to the basi- and partly to the 

 ex-occipital region. There is an interorbital septum, the 

 otic bones do not fuse into a periotic before uniting with 

 other elements, there is no separate parasphenoid, and the 

 complex mandible articulates with a quadrate bone. The 

 ankle-joint is between the proximal and distal tarsal bones. 

 The oval nucleated red corpuscles are smaller than those of 

 the Ichthyopsida, the heart possesses two auricles and one 



