xiii PHYLUM CHORDATA 20< 



o. 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 flesh v 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 opcrculum is always present, and is 

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

 tinued below into 1 a brancJiiostegal 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, 1^, jug.pl.) 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 



