FINS 69 



arranged, being alternately directed to the right or left, none of 

 them being truly vertical as in other fishes (Fig. 42E). The 

 closely related Soldier-fishes (Holocentrum) of the coral reefs of 

 tropical seas derive their name from the stout and sharply 

 pointed spines with which the fins are provided. In a few 

 fishes, notably in the Weever ( Trachinus) (Fig. 58A) and in the 

 Poison-fish {Synanceia) (Fig. 58B), the spines of the dorsal fin 

 are associated with poison glands, thus adding greatly to their 

 efficiency as defensive weapons {cf, p. 142). 



In the majority of fishes the dorsal and anal fins are capable 

 of being erected or depressed at will, the separate spines or 

 soft-rays being provided with special muscles for this purpose. 

 When the fish is moving at any speed, the fins are always 

 lowered, and in a fast-swimming form such as the Mackerel 

 {Scomber) both the dorsal and anal can be folded away into 

 more or less deep grooves in the body, the object being to 

 prevent them from breaking the streamline form and so im- 

 peding progress. In the allied Sail-fish {Istiophorus) the spinous 

 dorsal fin is of enormous size, and is believed to be projected 

 from the surface of the sea and used like a sail to assist progres- 

 sion, but the whole structure can be tucked away into a deep 

 groove when not required (Fig. 6a). After a rapid burst of 

 speed, most fishes erect the dorsal and anal fins to their fullest 

 extent, and these act as brakes and assist in slowing down. 



Among other fishes with more or less modified dorsal fins, 

 the Trigger-fishes and Bichirs are worthy of mention. The 

 Trigger-fishes (Balistidae) owe their name to the structure of 

 the spinous dorsal, this being supported by three spines, the 

 first very strong and hollowed out behind to receive a bony 

 knob at the base of the second; by this mechanism the first 

 spine remains immovably erect until the second, which acts as 

 a trigger, is depressed (Fig. 29B). The Bichirs {Polypterus) are 

 not really Acanthopterygian fishes, but the anterior part of the 

 dorsal fin takes the form of a number of separate, flag-like 

 finlets, each consisting of a stout spine supporting a sail-like 

 membranous flap (Fig. 28m), hence the name Polypterus (many 

 fins)! 



In the Mackerel [Scomber), Tunny (Thunnus), Bonito {Gym- 

 nosardd) , and allied forms the soft dorsal fin is followed by a row 

 of separate finlets, each of which is made up of a single much 

 branched ray (Figs. 4, 5A). Their function is a little obscure, 

 but they may act as subsidiary rudders. 



The Remoras or Sucking-fishes [Echeneididae) are remarkable 



