4U 



COMPARATIVE ANATOMY. 



of metameres that have been suppi'essed in the course of this 

 differentiation. Further differentiations are brought about by the 

 development of paired appendages. The hinder ones in the Gna- 

 thostomata separate the trunk and tail more distinctly from one 

 another, as do the anterior ones in the case of the head and trunk. 



When the anterior appendages are separated from the head, as 

 they are even in the Selachii among the Fishes, a cervical region 

 is differentiated from the trunk, and connects it with the head. 

 We meet with this arrangement in the Amphibia. The trunk is 

 affected by further differentiations, and in the Amniota is separated 

 into cervical, thoracic, and lumbar regions. 



The caudal portion of the body undergoes a gradual metamor- 

 phosis. In the Fishes it is scarcely marked off; in the Amphibia 

 (Urodela) and Reptilia (Saurii, Crocodilini) it is separated from the 

 trunk by the hinder pair of appendages, and by its diminished bulk. 

 Although it is atrophied in Birds, it is in the Mammals only that it 

 acquires the character of an appendage of the body, in consequence 

 of its great decrease in bulk, even though it may still be of some 

 length. 



Appendages. 



§317. 



We must divide the appendages which are given off from the 

 body of the Vertebrata, and which function principally as locomotor 

 organs, into paired and unpaired parts. The unpaired ones are 

 developed from a vertical membrane — a continuation of the integu- 

 ment — which extends from the head to the anus. When firm 

 structures and special muscles are developed in tliis membrane, this 

 purely dermal process is converted into a fin.' This organ either retains 



its pnmitive continuity 

 ^^^^S23fe=^ -^ of arrangement (Fig. 2 1 4, 



A), or by the atrophy of 

 some, and by the in- 

 creased development of 

 the remaining portions, 

 is broken up into several 

 parts. X These are dis- 

 tinguished according to 

 their position as dorsal, 

 caudal, and anal fins 

 {Fig.2U,Bdca). They 

 function chiefly as direct- 

 ing organs; the caudal 

 fin alone has any higher locomotor significance in so far as the 

 caudal portion of the body plays an important part in locomotion. 

 These organs, which are commonly found in Fishes, are also seen in 

 the early stages of development in the Amphibia, in some of which 



Fig. 214. Diagram of the unpaired fins. A Primi- 

 tive stage. B Diilerentiated stage, d Dorsal fixi 

 (d' Fatty fin) , c Caudal ; a Anal ; p Thoracic j 

 V Ventral fins. 



