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 (Yiphias) 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 (Syngnathus), the 
“Flute-mouths ” (/istularia), 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Teleosts the mouth can be protruded and withdrawn at will by a 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 lke 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 Gadidae, 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. 
