Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/684

666 666 ICHTHYOLOGY [GEOLOGICAL two different kinds from Cuba; and various species of Sjrfiyrcena, JBalistes, Ostracion, Caranx, Lftckiiolcemus, Tetragonurus, Thynmis, have been found to be poisonous in all seas between the tropics. All or nearly all these fishes acquire their poisonous properties from their food, which consists of poisonous Medusas and corals, or of decomposing substances. Frequently the fishes are found to be eatable if the head and intestines are removed immediately after capture. In the West Indies it has been ascertained that all the fishes living and feeding on certain coral banks are poisonous. In other fishes the poisonous properties are developed at certain seasons of the year only, especially the season of propagation ; as the barbel, pike, and burbot, whose roe causes violent diarrhoea when eaten during the season of spawning. Poison-organs are more common in the class of fishes than was formerly believed, but they seem to have exclusively the function of defence, and are not auxiliary in procuring f od, as in venomous snakes. Such organs are found in the sting-rays, the tail of which is armed with one or more powerful barbed spines. Although they have no special FIG. C6. Portion of tail, with spines, of Aetobatis ntirinari, a Sting-ray from the Indian Ocean, a, natural size of spine. organ secreting poison, or canal in or on the spine by which the venomous fluid is conducted, the symptoms caused by a wound from the spine of a sting-ray are such as cannot be accounted for merely by the mechanical lacera tion, the pain being intense, and the subse quent inflammation and swelling of the wounded part terminating not rarely in gangrene. The mucus secreted from the surface of the fish and inoculated by the jagged spine evidently possesses venomous properties. This is also the case in many Scorpsenoids, and in the weever (Trachinus), in which the dorsal and opercular spines have the same function as the caudal spines of the sting-rays ; in the weevers, however, the spines are deeply grooved, the groove being charged with a fluid mucus. In Synanceia the poison-organ (fig. 67) is still more developed : each dorsal spine is in its terminal half provided with a deep groove on each side, at the lower end of ,..,. i ji. which lies a pear-shaped bag containing the milky poison ; it is prolonged into a membranous duct, lying in the groove of the spine, and open at its point. The native fishermen, well acquainted poison-bags, of Synanceia ver- ,-ucosa (Indian Ocea &quot;)- FIG. 68. Thdlassophryne reticuJata. with the dangerous nature of these fishes carefully avoid handling them ; but it often happens that persons wading with naked feet in the sea step upon the fish, which gene FIG. 09. Oporculiir part of the poison-apparatus rf TfuOatsopftrynefPaataaA ). 1. Hinder half of the head, w ith the venom-sac* in situ, a, lateral line and its branches; 6, gill-opening; c, ventral fin; d, base of pectoral fin; e, base of dorsal. 2. Operculum ivith tlie P erf rally lies hidden in the sand. One or more of the erected spines penetrate the skin, and the poison is injected into the wound by the pressure of the foot on the poison-bags. Death has frequently been the result. The most perfect poison-organs hitherto discovered in fishes are those of Thalassophryne, a Batrachoid genus of fishes from the ..j^ .jj.u. coasts of Central 5 ^; i America. In these $ - ~-James500 (talk)&amp;lt; : James500 (talk) ~ r James500 (talk). fishes again tlu-|r ,, operculum and the %;&amp;gt;-? : - ._ two dorsal spines ^^^^ uBH^^^^^i^ are the weapons. |j The former (fig. GO) is very narrow, vertically styli- form, and very mobile; it is armed behind with a spine, eight lines long, and of the same form as the hollow venom-fang of a snake, being perforated at its base and at its extremity. A sac covering the base of the spine dis- 1 ;. Cliarges Its con- tents through the & apertures and the canal in the inte- rior of the spine. The structure of the dorsal spines is similar. There are no secretory glands imbedded in the membranes of the sacs, and the fluid must be secreted by their mucous membrane. The sacs have no external mus cular layer, and are situated immediately below the thick loose skin which envelops the spines to their extremity ; the ejection of the poison into a living animal, therefore, can only be effected, as in Synanceia, by the pressure to which the sac is subjected the moment the spine enters another body GEOLOGICAL DISTRIBUTION. Of what kind the fishes were which were the first to make their appearance on the globe, whether or not they were identical with or similar to any of the principal types existing at present, are questions which probably will for ever remain hidden in mystery and uncertainty. The supposition that the Pharyngobranchs and Cyclostomes, the lowest of the vertebrate series, must have preceded the other subclasses, is an idea which has been held by many zoologists, and, as the horny teeth of the Cyclostomes are the only parts which under favourable circumstances could have been preserved, palaeontologists have ever been searching for this evidence. Indeed, in deposits belonging to the Lower Silurian and Devonian, in Russia, England, and North America, minute, slender, pointed horny bodies, bent like a hook, with sharp opposite margins, have been found A and described under the name of &quot;conodonts.&quot; More frequently they possess an elongated basal portion, in which there is generally a larger tooth with rows of similar but smaller denticles on one or both sides of the larger tooth, according as this is central or at one end of the base. In other examples there is no prominent central tooth, but a series of more or less similar teeth is implanted 70 _ Rlght dent ^ p i a t e