138 



FIRST LESSONS IN ZOOLOGY. 



144). In this respect the backboned animals differ from 

 the backboneless or invertebrate animals, in which there is 

 but one body-cavity, with the nervous system usually situ- 

 ated on the floor of this cavity. 



Vertebrates have a true heart, with one, generally two, 

 auricles, and one or two ventricles, and, besides arteries and 

 veins, a system of capillary vessels, which are minute tubes 

 connecting the ends of the smaller arteries with the smaller 

 veins. There are no genuine capillaries in the lower ani- 

 mals exactly comparable with those of vertebrates. 



FIG. 145. A diagrammatic section across the body in the chest region, x. the 

 neural canal, which contains the spinal cord; the black mass surrounding it 

 is a vertebra; a, the gullet, a part of the alimentary canal: ft, the heart; sy, 

 sympathetic nervous system; 1 1, lungs; the clotted lines around them are the 

 pleurae; rr, ribs; st, the breastbone. 



The blood is red in all the vertebrates except the lance- 

 let, and besides red, contains white corpuscles. While 

 fishes and tadpoles breathe by gills, all land and amphibious 

 vertebrates breathe the air directly by lungs connected by a 

 windpipe (trachea] with the mouth. The nervous system 

 consists of a brain and spinal cord. The brain consists of 

 four pairs of lobes, i.e., the olfactory lobes, cerebral hemi- 

 spheres, the optic thalami with the pineal gland, and the 

 optic lobes; besides these lobes, which are arranged in pairs, 

 there are two single parts of the brain, the cerebellum and 

 the beginning of the spinal cord, called the medulla ob~ 

 longata. 



