PART II. 

 SYSTEMATIC ZOOLOGY. 



SUB-PHYLU.M VERTEBRATA. 



^Ietameric metazoan animals with a complete alimentary 

 canal, the anterior portion of which, at least in embryonic 

 stages, is provided with gill slits. The central ner\'ous system, 

 consisting of brain and spinal cord, is hollow, and is situated 

 entirely on one side of the alimentary tract. Between alimen- 

 tary canal and central nervous system is a skeletal axis — the 



Fig. 222. Anatomy of a vertebrate, based on ^/«(>/)'j/ow«. a, anus; i5, brain; 

 ^i, heart ; /, lung ; li, liver ; i , ovary : od, oviduct ; c/, ostium tubse ; /, pelvis ; s, 

 stomach ; st, sternum ; /, tongue ; /;-, trachea ; u, urinary bladder. 



notochord — of entodermal origin, which mav persist throughout 

 life. Around this notochord is developed a segmented skeleton, 

 •consisting of skull and vertebral column. The heart is on the 

 .abneural side of the alimentary canal, and consists of at least 

 two chambers. It connects with a dorsal aorta bv arterial 

 arches, which, however, may become greatly reduced and modi- 

 fied in the adult. The blood contains red corpuscles in addition 

 to leucocytes. The reproduction is entirely sexual. The verte- 

 irates are free throughout their entire existence. 



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