xm PHYLUM CHORDATA 209 



3. GENERAL ORGANISATION. 



External Form. The typical form of the Teleostomi is very 

 fairly represented by that of the Trout (Fig. 806) a long, com- 

 pressed body, nearly half of which is formed by the tail, pointed 

 anterior and posterior ends, a large vertical tail-fin, a head of mode- 

 rate 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. 824) ; 

 in the allied Polyodon the snout takes the form of a horizon- 

 tally 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. 825). 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. 830, 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. 824 and 828, &.). An operculum is always present, and is 

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

 tinued below into a branchiostegal membrane (Fig. 807, br. m.), 

 which, except in Crossopterygii and the Sturgeons, is supported 

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

 823, 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. 826, B, jug. pi.) in the same position. Spiracles are 

 present only in Polypterus (Fig. 838) and some Sturgeons. 



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



VOL. II P 



