354 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 spinosus) 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 either 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 Fishes 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 (Siphonostoma typhle), 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 lateral 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, Tetrodon and Orthagoriscus)* in connexion 
with both the dorsal and anal fins, but in these Fishes the 
1 Bridge, Journ. Linn. Soc. (Zool.), xxv. 1896, p. 530. 
