218 ZOOLOGY SECT. 



moderate size, and a terminal mouth. Such a form is eminently 

 fitted for rapid progression through the water. But from this 

 characteristic fish-form there are many striking deviations. The 

 body may be greatly elongated and almost cylindrical, as in the 

 Eels ; or of great length and strongly flattened from side to side, 

 as in the Ribbon-fishes ; or the head may be of immense proportional 

 size and strongly depressed, as in certain shore-fishes, such as the 

 " Fishing-frog " ; or, as in the beautiful Reef -fishes, the whole 

 body may be as high as it is long. The mouth sometimes has a 

 ventral position, as in Elasmobranchs, with the snout prolonged 

 over it. This is the case, for example, in the Sturgeons (Fig. 884) ; 

 in the allied Polyodon the snout takes the form of a horizontally 

 flattened shovel-like structure, about one-fourth the length of the 

 body. On the other hand, in the ground-feeding " Star-gazers " 

 and some other Acanthopteri the lower jaw is underhung like 

 that of a bull -dog, and the mouth becomes dorsal in position. 

 A beak may be produced by the prolongation of the upper jaw, 

 as in the Sword-fish, or of the lower jaw, as in the Half-beak or 

 Gar-fish, or of both jaws, as in the Bony Pike (Fig. 885). Such a 

 projection is not to be confounded with the snout of the Sturgeon 

 or Polyodon, being formed by the elongation of the bones of the 

 jaws (premaxilla, maxilla, dentary, &c.), whereas in the two 

 Chondrostean forms referred to it is the anterior region of the 

 cranium which is prolonged. Still another form of " snout " 

 is produced in many Teleostei by the great mobility of the jaws, 

 allowing of their protrusion in the form of a short tube. In the 

 Wrasses or " lip-fishes " the mouth is bounded by fleshy lips 

 (Fig. 890, lp.). 



Tactile processes or barbels sometimes arise from the head ; the 

 most familiar example is that on the chin of the Cod and Haddock 

 (Figs. 884 and 888, 6.). An operculum is always present, and is 

 supported by a variable number of investing bones ; it is con- 

 tinued below into a branchiostegal membrane (Fig. 866, br. m.), 

 which, except in Crossopterygii and the Sturgeons, is supported 

 by bony rays. In Polypterus a pair of bony jugular plates (Fig. 

 883, B, jug. pi.) are placed at the lower end of the branchiostegal 

 membrane, between the rami of the mandible : Amia has a single 

 plate (Fig. 886, B, jug. pi.) in the same position. Spiracles are 

 present only in Polypterus (Fig. 898) and some Sturgeons. 



The commonest number of median fins is two dorsals, one caudnl, 

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

 (Figs. 888 and 890), or there may be a continuous median fin 

 extending along the back and round the end of the tail to the anus. 

 The dorsal fin is sometimes partly or wholly represented by a 

 series of small finlets (Fig. 883). The tail fin may be diphycercal, 

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

 progression. But in the Sea-horse (Fig. 892) there is no caudal fin, 



