VERTEBRATA 



19 



coracoid, clavicle, and epiclavicle. There is sometimes an 

 interclavicle. The dermal portion of the fins may include 

 as supports certain " fin-rays," or it may include only hair- 

 like rods or " actinotrichia." The latter characterize 

 primitive types. In the case of the median fins it is char- 

 acteristic of primitive fishes to have the baseosts and 

 axonosts articulated with the neural spines of the vertebia'. 

 In modern fishes the axonosts are not so articulated, and do 

 not correspond with them, while the baseosts are rudi- 

 mental or wanting. (Fig. 3.) 



Ac 



/ 



/ 

 / 



D 



/ / 



B 



D 



D 



rr 



Ba 

 Ak 

 /Vs. 



FxG.3. — Diagrams ot actinopliores; A, eutotetramerous; B^ ectetramerous; C, />. 

 ectrimerous; E^ ecdimeroas; Ac, actinotrichia; i^r, fin-rays; Ba, baseost; Ax. axu- 

 nost; Ic, intercentrum. 



Remarkable modifications take place in the successive 

 evolution of the caudal fin. The primitive condition of the 

 vertebral column in the tail of a fish is straight, with spines 

 equally divergent above and below. This is the diphycercal 

 tail (Fig. 4, A). It persists in some modern fishes, e. g., 

 the eels. In the next stage the axis is turned up at the 

 end, and the spines of the inferior side are spread out fan- 

 like, while those of the superior side are crowded together. 

 The dermal part of the fin may develop an angle on the 

 inferior spines (haemal spines), the result being a two-lobed 

 fin, in which the upper lobe is much larger than the lower. 



