J 



226 ZOOLOGY sect. 



some Sliaiks and of Holocephali (Fig. 776, (l.J. v. 1 i>ct,f. r. 1). In 

 Polypterus (Fig. 872) each finlet is supported along its anterior 

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



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



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



{Lojiliiiis) its foremost rays are elongated and bear lobes or lures by 



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



p In the Sucking-fish (Echeneis) the anterior dorsal 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. 872) 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 {Exocoetus, Dactylc/pterus) 

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

 its long fiying leaps into the air. In the Butterfly-fish {Gasterochisma) 

 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. 872-875) and 

 in the Physostomi (Fig. 854 and 855), in wiiich 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. 879, ^w./.), when their position is called 

 thoracic, or on the throat (Fig. 877), when they are said to be 

 jugular in position. 



A very remarkable deviation from the typical form occurs in the 

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

 (Fig. 882) 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 

 pure white, the upper side dark. The eyes (r.e, he) are both 

 on the upper or dark-coloured side, and the skull is distorted so as 

 to adapt the orbits to this change of position. The abdominal 

 cavity is very small, the anus placed far forward, and the dorsal 

 and ventral fins are sometimes continuous. Young Flat-fishes 

 swim in the ordinary vertical position, but after a time they 

 lie on one side and assume the adult i^eculiarities, the eye on the 

 lower side gradually rotating until it reaches the upper surface. 



