296 



CLASSIFICATION OF ANIMALS 



epidermis. There are typically two pairs of appendages but 

 these have' been lost in many of the Squamata. The heart has 

 two auricles and the ventricle is partly divided by an incom- 

 plete partition. The lungs are spongy in structure. There 

 are twelve cranial nerves. This comprises the ten cranial 

 nerves of the amphibia and the first spinal nerve as well as a 

 spinal accessory nerve not represented as a separate nerve in 



arn.c 



CO. 



FIG. 174. Diagram of a bird embryo with the foetal membranes, the amnion 

 and the allantois. am 1 , inner or true amnion; aw 2 , outer or false amnion; am.c, 

 amniotic cavity; al, allantois. Other lettering as in the preceding figure. 



Amphibia. The cerebral hemispheres are well developed. The 

 intestine, ureters and genital ducts open into a cloaca. The 

 eggs are fertilized in the oviduct. They are very large and are 

 covered by a tough shell secreted by the oviduct. The eggs 

 are generally not brooded but are buried in the earth and allowed 

 to develop at atmospheric temperature. The embryo is pro- 

 vided with the fcetal membranes, the amnion and allantois. 



