TAXONOMY, ANATOMY, EMBRYOLOGY 



ure 10 ) . In one, leading toward the modern Amphibia, the hypocentrum 

 gradualh' replaces the pleurocentrum. In the other, leading to the reptiles, 

 the pleurocentrum gradually replaces tlie hypocentrum. This replacement 

 is not yet complete in the Rhyncocephalia, the most primitive order of 

 living reptiles. 



The pre- and post-zygopoph)'ses, articular processes on the neural 

 arches, first appear among the Amphibia. They strengthen the joints be- 

 tween successive vertebrae. Another innovation is the addition of two 

 more differentiated types of vertebrae in addition to the trunk and caudal 

 vertebrae. One or two cervical vertebrae serve to make a freely movable 

 joint between the skull and the vertebral column, a character of adaptive 

 value to land-dwellers, but not to the strictly aquatic fishes. Between the 

 trunk and caudal regions is a single sacral vertebra specialized by fusion 

 with its ribs to attach the pelvic girdle to the axial skeleton. 



The vertebrae of the Reptilia differ from those of the Amphibia mainly 

 in that the centrum is a pleurocentrum rather than a hypocentrum (in 

 extant forms). Also, the number of cervical and sacral vertebrae is in- 

 creased. In the cervical region, the ribs are now fused to the vertebrae 

 (Figure 11). The vertebral column of the birds differs from that of the 

 reptiles chiefly in that a highly flexible articulation has been developed 

 between the cervical vertebrae, and those of the body are largely fused. 

 The caudal vertebrae are much reduced (Figure 12). The vertebral col- 

 umn of the mammals differs from that of the reptiles mainly by the de- 

 velopment of curvatures associated with the mechanics of locomotion 

 (Figure 13). 



Thus, a survey of the vertebral columns of the extant classes of verte- 

 brates leads back to the statement with which this discussion was opened, 

 that, within a single phylum, any particular system seems to be based 

 upon a prototype which is simply varied from class to class. 



Serial Homotogy in the Crustacea. Structures, which are similar in 

 different species because of common inheritance, irrespective of the di- 

 verse uses to which they may be put, are called homologous structures. 

 There is no group of living things which does not present homologies in 

 all of its structural systems, but perhaps no set of homologous structures 



/Last thoracic vertebra 

 .Lumbar vertebrae 



-/Sacrum 



• Pelvic girdle 



Figure 13. The Skeleton of a Primitive Mammal, the Tree Shrew, Tupaia. (After 

 Gregory, from Romer, "The Vertebrate Body," 2nd Ed., W. B. Saunders Co., 1955.) 



55 



