B. Tetrodontina.—Tail and caudal fin distinct. Part of the œsophagus much distensible, and capable of being filled with air. No pelvic bone.
“Globe-fishes” have a short, thick, cylindrical body, with well developed fins. It is covered with thick scaleless skin, in which, however, spines are imbedded of various sizes. The spines are very small, and but partially distributed over the body in some species, whilst in others they are very large, and occupy equally every part of the body. These fishes have the power of inflating their body by filling their distensible œsophagus with air, and thus assume a more or less globular form. The skin is, then, stretched to its utmost extent, and the spines protrude and form a more or less formidable defensive armour, as in a hedgehog; therefore they are frequently called “Sea-hedgehogs.” A fish thus blown out turns over and floats belly upwards, driving before the wind and waves. However, it is probable that the spines are a protection not only when the fish is on the surface and able to take in air, but also when it is under water. Some Diodonts, at any rate, are able to erect the spines about the head by means of cutaneous muscles; and, perhaps, all fill their stomach with water instead of air, for the same purpose and with the same effect. In some Diodonts the spines are fixed, erect, not movable. The Gymnodonts generally, when taken, produce a sound, doubtless by the expulsion of air from the œsophagus. Their vertebral column consists of a small number of vertebræ, from 20 to 29, and their spinal chord is extremely short. All these fishes have a bad reputation, and they are never eaten; indeed, some of them are highly poisonous, and have caused long continued illness and death. Singularly, the poisonous properties of these fishes vary much as regards intensity, only certain individuals of a species, or individuals from a certain locality, or caught at a certain time of the year, being dangerous. Therefore it is probable that they acquire their poisonous quality from their food, which consists in corals and hard-shelled Mollusks and Crustaceans. Their sharp beaks, with broad masticating posterior surface, are admirably adapted for breaking off branchlets of coral-stocks, and for crushing hard substances.
Fig. 311.—Jaws of Tetrodon.
Fig. 312.—Tetrodon margaritatus.
Tetrodon (including Xenopterus).—Both the upper and lower jaws are divided into two by a mesial suture.
Extremely numerous in tropical and sub-tropical zones, more than sixty species being known. In some of the species the dermal spines are extremely small, and may be absent altogether. Many are highly ornamented with spots or bands. A few species live in large rivers—thus T. psittacus from Brazil; T. fahaka, a fish well known to travellers on the Nile, and likewise abundant in West African rivers; T. fluviatilis from brackish water and rivers of the East Indies. The species figured is one of the smallest, about six inches long, and common in the Indo-Pacific.
Diodon.—Jaws without mesial suture, so that there is only one undivided dental plate above and one below.
In these fishes, as well as in some closely allied genera, the dermal spines are much more developed than in the Tetrodonts; in some the spines are erectile, as in Diodon, Atopomycterus, Trichodiodon, and Trichocyclus; in others they are stiff and immovable, as in Chilomycterus and Dicotylichthys. Seventeen species are known, of which Diodon hystrix is the most common as it is the largest, growing to a length of two feet. It is spread over the Tropical Atlantic as well as Indo-Pacific, as is also a smaller, but almost equally common species, Diodon maculatus.