62 GENERAL ZOOLOGY 



atlas vertebra, in higher forms consists of a number of vertebrae 

 characterized by a reduction in the size of ribs or by the complete 

 absence of ribs. In the higher forms the cervical region is 

 followed by the thoracic region, provided with well-developed 

 ribs; the lumbar region, usually without ribs; the sacral region, 

 made up of several vertebrae, fused together and firmly united 

 to the pelvic girdle; and the caudal or tail region (Fig. 36). The 

 caudal region is sometimes reduced or modified as in the case of 

 the pygostyle of birds (Fig. 44), or the coccyx of man (Fig. 63). 



Fig. 36. — Regions of the vertebrate skeleton (cat). (From Jayne, Mammalian 

 Anatomy, J. B. Lippincott & Co. By permission.) 



Unpaired Appendages. — The dorsal, caudal, and anal fins of 

 fishes and the continuous median fin of larval amphibians, such as 

 the tadpole of the frog, are examples of unpaired appendages. 

 They are located in the midplane of the body, and in fishes are 

 supported by cartilaginous or bony spines. In amphibians 

 skeletal elements are lacking. Median fins, by extending the 

 lateral surface of the trunk and tail, enlarge the pushing surface 

 of the trunk and tail and thus increase the efficiency of the 

 swimming strokes. 



Notochord. — In the lowest vertebrates, the Cyclostomata, 

 there is a rudimentary vertebral column with poorly developed 

 neural arches. The centra of the vertebrae are absent and in 

 their place is a long unsegmented cylindrical rod, the notochord, 

 which serves as a body axis. The notochord develops in all 

 vertebrate embryos (Fig. 37). In fishes, vertebrae are devel- 

 oped, but the notochord is not completely replaced even in the 



