The Sawfishes.—A certain genus of rays (Pristis, the sawfish) and a genus of sharks (Pristiophorus, the saw-shark), possess a similar spatula-shaped snout. But in these fishes the snout is provided on either side with enamelled teeth set in sockets and standing at right angles with the snout. The animal swims through schools of sardines and anchovies, strikes right and left with this saw, destroying the small fishes, who thus become an easy prey. These fishes live in estuaries and river mouths, Pristis in tropical America and Guinea, Pristiophorus in Japan and Australia. In the mythology of science, the sawfish attacks the whale, but in fact the two animals never come within miles of each other, and the sawfish is an object of danger only to the tender fishes, the small fry of the sea.
Fig. 153.—Saw-shark, Pristiophorus japonicus Günther. Specimen from Nagasaki.
Peculiarities of Jaws and Teeth.—The jaws of fishes are subject to a great variety of modifications. In some the bones are joined by distensible ligaments and the fish can swallow other fishes larger than itself. In other cases the jaws are excessively small and toothless, at the end of a long tube, so ineffective in appearance that it is a marvel that the fish can swallow anything at all.
In the thread-eels (Nemichthys) the jaws are so recurved that they cannot possibly meet, and in their great length seem worse than useless.
In some species the knife-like canines of the lower jaw pierce through the substance of the upper.
In four different and wholly unrelated groups of fishes the teeth are grown fast together, forming a horny beak like that of the parrot. These are the Chimæras, the globefishes (Tetraodon), and their relatives, the parrot-fishes (Scarus, etc.), and the stone-wall perch (Oplegnathus). The structure of the beak varies considerably in these four cases, in accord with the difference in the origin of its structures. In the globefishes the jaw-bones are fused together, and in the Chimæras they are solidly joined to the cranium itself.
The Angler-fishes.—In the large group of angler-fishes the first spine of the dorsal fin is modified into a sort of bait to attract smaller fishes into the capacious mouth below. This structure is typical in the fishing-frog (Lophius), where the fleshy tip of this spine hangs over the great mouth, the huge fish lying on the bottom apparently inanimate as a stone. In other related fishes this spine has different forms, being often reduced to a vestige, of little value as a lure, but retained in accordance with the law of heredity. In a deep-sea angler the bait is enlarged, provided with fleshy streamers and a luminous body which serves to attract small fishes in the depths.
The forms and uses of this spine in this group constitute a very suggestive chapter in the study of specialization and ultimate degradation, when the special function is not needed or becomes ineffective.
Similar phases of excessive development and final degradation may be found in almost every group in which abnormal stress has been laid on a particular organ. Thus the ventral fins, made into a large sucking-disk in Liparis, are lost altogether in Paraliparis. The very large poisoned spines of Pterois become very short in Aploactis, the high dorsal spines of Citula are lost in Alectis, and sometimes a very large organ dwindles to a very small one within the limits of the same genus. An example of this is seen in the poisoned pectoral spines of Schilbeodes.