210 ZOOLOGY SECT. 



and one ventral, but this number may be increased or diminished 

 (Figs. 828 and 830), or there may be a continuous median fin ex- 

 tending along the back and round the end of the tail to the vent. 

 The dorsal fin is sometimes partly or wholly represented by a 

 series of small finlets (Fig. 823). The tail-fin may be diphycercal, 

 heteroeercal, or homocercal, and is usually the chief organ of 

 progression, but in the Sea-horse (Fig. 832) there is no caudal fin, 

 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. 



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

 the Trout, but in the Acanthopteri and Pharyngognathi more 

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

 fins are unjointed, forming spines I (Figs. 829 and 830, d. /.), some- 

 times large and strong enough to recall the dermal defences of 

 some Sharks and of Holocephali (Fig. 827, d.f. r.\,pct.f. r.I). In 

 Polypterus (Fig. 823 ) 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 

 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 (JEcheneis) 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. 823) 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 Elasmobranchs and Holocephali. The pectorals 

 vary considerably in size, and in the Flying-fish (JExoc&tus) form 

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

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

 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-cleft, but the pelvics always become more or less shifted 

 forwards from their typical position beside the vent. The change 

 in position is least in the three "ganoid" orders (Figs. 823-826) and 

 in the Physostomi (Figs. 806 and 827), in which they are usually 

 between the middle of the abdomen and the vent, 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. 83Q,pv.f.), when their position is called 



