154 FISHES CHAP. 



vertical gape of the mouth is greatly increased, but in a few 

 Teleosts a beak may result from a forward extension of one jaw 

 only, the upper in the Sword-Fish (Xipliias) and the lower in 

 the " Half-Beak " (Hemirhamphus). A further modification is to 

 be noted in many Teleosts, in which, owing to the forward pro- 

 longation and inclination of the skeletal supports of the jaws, 

 the mouth is at the extremity of a longer or shorter spout-like 

 beak, and is then usually very small. This is the case in the 

 " Sea-Horse " (Hippocampus), the Pipe-Fishes (Syncjnathus), the 

 " Flute-mouths " (Fistularici), and the Trumpet-Fish (Centriscus), 

 and especially in certain species of the African family Mormy- 

 ridae, where the pore-like mouth is at the extremity of a long, 

 tapering, downwardly -curved proboscis (Fig. 330). In many 

 Teleosts the mouth can be protruded and withdrawn at will by a 

 sliding motion of the bones of the upper jaw (premaxillae) on the 

 anterior skull bones by which they are supported. From this 

 point of view the toothless mouth of the Sturgeon is even more 

 remarkable. By a forward or a backward swing of the elements 

 which form the upper half of the hyoid arch (hyomandibular and 

 symplectic) the mouth can be thrust downwards from the under 

 side of the head like a spout, when the Fish is feeding, and subse- 

 quently retracted. In not a few Fishes the forepart of the head 

 is prolonged forwards over the mouth and jaws in the form of a 

 rostrum or " snout " ; it is, in fact, to the growth of a snout that 

 the ventral position of the mouth in Fishes is generally due. This 

 feature is more or less characteristic of most Elasmobranchs, in 

 which the snout forms a cut-water overhanging the mouth. In 

 the Holocephali the snout is short and blunt, except in Harriotta, 

 where it is pointed and unusually long. Among the Chondrostei 

 the Sturgeon has an exceptionally massive snout, the length and 

 shape of which differs in different species. In the allied Polyodon 

 the thin, flattened, spoon-like snout is scarcely less than one- 

 fourth the length of the body (Fig. 289). 



Simple or branched tactile filaments or " barbels " are present 

 on different parts of the head in many Teleostomi, sometimes at 

 or near the chin, as in certain G-adidae, like the Haddock and 

 Cod, or on the under surface of the snout, in front of the mouth, 

 as in the Sturgeon. In the Siluridae (Fig. 356), where they are 

 found in relation with the upper and lower jaws, and even between 

 the nostrils, these structures are often remarkably developed. 



