APPENDAGES OF THE VERTEBRATA. 415 



(many Urodela) they are indeed permanently present, but supporting 

 organs are not developed in tliem. 



In the Reptilia indications of the vertical dermal fringe can 

 sometimes be just made out, but in most it is altogether absent, as 

 it is also in the higher classes ; the vertical fin-like structures seen 

 in many of the Cetacea must be regarded as organs which have been 

 acquired by that order independently. The same remark applies to 

 the horizontal caudal fin of these Mammals. 



§ 318. 



Unlike the arrangement seen in many divisions of the Inverte- 

 brata, where paired appendages are found on all, or at least on a 

 large number of metameres, they are — and, so far as we know, with- 

 out one exception — confined to an anterior and a posterior pair in 

 the Vertebrata. 



They appear to be homodynamous organs, which gradually get 

 to vary greatly in form in correlation with their great variety of 

 function. They are probably derived from metamorphosed respira- 

 tory appendages of the head (branchial arches and rays), so that 

 they are not absolutely new arrangements. 



They are absent in the Acrania and Cyclostomata, but are 

 generally present in the Gnathostomata. Although in some divisions 

 of these latter the appendages are atrophied, this atrophy is in every 

 case a secondary arrangement, which presupposes the fully-developed 

 condition. This is proved by the various stages of atrophy, which may 

 be seen in the appendages and in the parts of which they are composed. 



In the lower condition seen in Fishes the appendages appear each 

 to form a single whole, undivided by external jointing into a number 

 of parts, while their increased surface is of importance as bearing on 

 the directing function of the organ. The anterior and posterior 

 appendages, which are here known as thoracic and ventral fins, 

 have essentially the same structure, although the thoracic fins are, 

 as a rule, much larger, in consequence of their position in the larger 

 part of the body. Their more powerful structure may be also ex- 

 plained by the fact that they take the initiative, and are consequently 

 of greater functional importance than the hinder appendages. 



Owing to their similar mode of aquatic progression the ap- 

 pendages of the fossil Enaliosaurii, as shown by the remains of their 

 skeletons, resembled the fins of Fishes — at any rate in the absence 

 of any transverse segmentation. 



Among the Amphibia we find the appendages transversely seg- 

 mented, for now several parts are sharply marked off from one 

 another. In the fore-limb we divide these into upper arm, forearm, 

 and hand ; to which the thigh, leg, and foot correspond in the hind- 

 limb. This division is correlated with the greater elongation of the 

 two first segments, which stand, in relation to one another, as the 

 arms of a lever, and are therefore set at an angle to one another. 



In addition to the differentiation herein implied, the terminal 



