ii6 



ELEMENTS OF BIOLOGY 



the zoologist because it contains in rudimentary form many of the 

 characters of the higher animals. The type form usually given is 

 Amphioxus. 



Sub-phylum IV. \^ertebrata (Fig. 65). Members of this phylum 

 have a notochord during their early existence, but in later life it is 



VERTEBRATA 



PLEURAL CAVITy 



SPINAL COfID 



HINDBRAIN 



MIDBRAIN 



FORiBRAIN 



ORAL CAVITY 



EXCRETORY ORGAN 

 BLADDER 



THACHEA 



HEART 



VISCERAL curr 



PERICARDIAL CAVITY 



•REPRODUCTIVE DUCT 



VERTEBRAE 



FIG. 65. A SCHEME OF VERTEBRATE ORGANIZATION 



Fig. 65. — Scheme of organization of Vertebrata. When lungs are present, as in 

 Amphibia, Reptilia, Aves, and Mammalia, they communicate with the floor of the 

 anterior region of the digestive canal by way of the trachea. Water breathers 

 expel water through the visceral or branchial clefts, indicated in the figure. In 

 mammals the lung cavities are separated from the pericardial cavity and the 

 peritoneal region of the coelom. These partitions are indicated by horizontal and 

 perpendicular planes. 



more or less completely replaced by a series of spool-like bones or 

 cartilages forming what is known as the vertebral column. True 

 skin composed of two layers, bony teeth, bone, hair, and feathers, 

 are found only in the Vertebrata, although, of course, not all Ver- 

 tebrata have all these characters. All Vertebrata exhibit segmenta- 

 tion, have red blood, are bilaterally symmetrical, have two pairs of 



