14 



PROCEEDINGS OF THE NATIONAL MUSEUM 



Elongate perciform fishes usually move forward by undulation, but 

 this may be by two very distinct methods. In one, the fish holds its 

 body more or less rigid and undulates the dorsal and anal fins only. 

 In these forms, the dorsal and anal soft fins tend to be long and the 

 fin rays to be closely spaced (i.e., two or more per vertebra), inserted 

 basally on a sort of ball and socket axis, and with well-developed 

 musculature. Locomotion by means of fin undulation seems to provide 

 precision of movement rather than speed and enables the fish to move 

 backward or forward with almost equal ease. Such a method of loco- 

 motion has been developed frequently among the lower teleosts; e.g., 

 gymnarchids, gymnotids, probably halosaurids and macrourids, and 

 the Syngnathiformes. It occurs, however, only in the ophidioids among 

 the Perciformes (fig. 16), and in the Tetraodontiformes. 



Flop the 

 dorsal and anal 



MOLIDAE 



Deep-bodied Fishes 



^ Undulate fins 

 separately fron 

 the body 



__-— ~BR° VUW 



Elongate Fishes 



CEPOLIDAE 



Undulate body and 

 vertical fins together 



Figure 3. — Diagram of certain types of forward motion in the perciform fishes and their 



derivatives. 



The other, more usual method of locomotion among elongate perci- 

 form fishes and their derivatives is for the fish to undulate its body and 

 fins together. Here, the vertical fins tend to coordinate their structure 

 as well as movement with that of the body, the relationship between 

 soft dorsal and anal fin rays and vertebrae becoming 1:1. Generally, 

 also, the number of vertebrae in such fishes is increased over the basal 

 percoid number of 24 or 25. This development of a 1:1 ratio between 

 soft fin rays and vertebrae in elongate perciform fishes occurs again and 

 again (fig. 16). Sometimes it occurs in free-swimming forms like the 

 Cepolidae or Schindleriidae, but more frequently it develops in bottom- 

 resting forms. 



