3 54 FISHES CHAP. 



some Fishes it would seem that the pectoral fins may assist loco- 

 motion by acting as paddles. The 15-spined Stickleback {Gast- 

 rosteus sjjinosus) frequently progresses by their aid alone ; and, as 

 their action can be reversed at pleasure, it is not unusual to see 

 this Fish move backwards. The fins appear to be rotated or 

 twisted in spiral movements like the tail when used for swim- 

 ming, or like the wings of Insects in flying. 



It has been mentioned that the function of the median fins 

 (dorsal and anal) is to give stability to the Fish by acting as 

 dorsal and ventral keels. This is certainly the case in the 

 generality of Fishes. Nevertheless, there are exceptional in- 

 stances in which one, or even both, of these fins are important 

 swimming organs, acting eithet as a substitute for a tail which 

 has become adapted for other uses, or as supplementary to that 

 organ. Thus, in some of the Syngnathidae (Pipe-Fishes and Sea- 

 Horses) the small size or absence of the caudal fin, and its use as 

 a prehensile organ, renders the tail of little or no value as a pro- 

 pelling organ : hence it is that these Tishes swim by a lateral 

 undulating movement of the dorsal fin. To enable them to do 

 this the supporting skeleton presents certain interesting modifica- 

 tions. In the majority of Teleosts the arrangement of the fin- 

 muscles, and the nature of the articulation between the dermal 

 fin-rays and their basal radial supports, which is generally some 

 form of a hinge-joint, are such as to limit the motion of the rays 

 to simple elevation or depression in the vertical plane, and no 

 lateral motion of the fin is possible. But in the Syngnathidae, 

 as in the Pipe -Fish {Siphono stoma tyjiUe), there is an excep- 

 tionally mobile articulation between the dermal fin-rays and 

 the distal radial nodules which their cleft bases embrace and the 

 bony proximal or basal radials, so that the fin can be flexed or 

 bent to the right or to the left. In addition to this, by a 

 change in the insertion of their tendons, the muscles correspond- 

 ing to the ordinary elevator and depressor muscles of the fin-rays 

 in other Fishes are capable of producing extensive latei-al move- 

 ments of the fin, or, by contracting in orderly sequence, of bringing 

 about the characteristic undulating motion .of the fin. A similar 

 mechanism exists in many Plectognathi {e.g. species of Balistes, 

 Monacanthus, Diodon, Tetroclon and Orthagorisms) ^ in connexion 

 with both the dorsal and anal fins, but in these Fishes the 



' Bridge, Journ. Linn. Soc. (ZooL), xxf. 1896, p. 530. 



