xm PHYLUM CHORDATA 219 



and the tail is prehensile, being used in the position of rest to coil, 

 in the vertical plane, round sea- weeds, &c. : when swimming it 

 hangs downwards, having no lateral movement, and locomotion is 

 effected by the vibration of the dorsal fin. 



The dermal-rays of the caudal fin are always jointed, as in the 

 Trout, but in most of the Acanthopteri and Pharyngognathi 

 more or fewer of the foremost rays of the dorsal, ventral, and pelvic 

 fins are unjointed, forming spines (Figs. 889 and 890, d. /.), some- 

 times large and strong enough to recall the dermal defences of 

 some Sharks and of Holocephali (Fig. 858, d.f. r. 1, pctf. r. /). In 

 Polypterus (Fig. 883) each finlet is supported along its anterior 

 edge by a strong spine, to which the soft rays are attached. 



The anterior dorsal fin may attain an immense size, and is 

 subject to some curious variations. In the Fishing-frog or Angler 

 (Lophius) its foremost rays are elongated and bear lobes or lures 

 by which small fishes are attracted as to the bait on a fishing-line. 



In the Sucking-fish (Echeneis) the anterior dorsal fin is modified 

 into an adhesive disc by means of which the fish attaches itself 

 to the bodies of Sharks and Turtles. 



The portion of the paired fins visible externally is usually very 

 thin, and supported entirely by dermal rays. But in the Crosso- 

 pterygii (Fig. 883) the rays form a fringe round a thick basal 

 lobe, which is supported by endoskeletal structures (vide infra). 

 This condition of things forms an approach to the structure met 

 with in Elasmobranchii and Holocephali. The pectorals vary con- 

 siderably in size, and in the Flying-fishes (Exoccetus, Dactylopterus) 

 form large, wing-like expansions, capable of sustaining the animal in 

 its long flying leaps into the air. In the Butterfly-fish (Gastrochisma) 

 the pelvic fins are similarly modified. In many Fishes the pelvics 

 are reduced to filaments or scales, and in some cases a sucking-disc 

 is developed in connection with them. The pectorals always retain 

 their normal position, just behind the gill-clefts, but the pelvics 

 often become more or less shifted forwards from the typical position 

 beside the vent. The change in position is least in the three 

 " ganoid " orders (Figs. 883-886) and in the Physostomi (Figs. 

 865 and 866), in which they are usually between the middle of 

 the abdomen and the anus, and are said to be abdominal in position ; 

 but in a large proportion of the fishes in the remaining orders 

 of Teleostei they come to be placed almost beneath the pectorals 

 (Fig. 890, pv. /.), when their position is called thoracic, or on the 

 throat (Fig. 888), when they are said to be jugular in position. 



A very remarkable deviation from the typical form occurs in the 

 Flat-fishes (Pleuronectidae), a family of Anacanthini. The body 

 (Fig. 893) is very deep and strongly compressed : the fish habitually 

 rests on the bottom, in some species on the right, in others on the 

 left side, partly covering itself with sand, and occasionally swimming 

 with a curious undulating movement. The under side is usually 



