The most perfect poison-organs hitherto discovered in fishes are those of _Thalassophryne_, a Batrachoid genus of fishes from the coasts of Central America. In these fishes the operculum again and the two dorsal spines are the weapons. The former (Fig. 100, ^2) is very narrow, vertically styliform 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 discharges its contents through the apertures and the canal in the interior 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 are without an external muscular layer, and 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. Finally, a singular apparatus found in many Siluroids may be mentioned in connection with the poison-organs, although its function is still problematical. Some of these fishes are armed with powerful pectoral spines and justly feared on account of the dangerous wounds they inflict; not a few of them possess, in addition to the pectoral spines, a sac with a more or less wide opening in the axil of the pectoral fin; and it does not seem improbable that it contains a fluid which may be introduced into a wound by means of the pectoral spine, which would be covered with it, like the barbed arrow-head of an Indian. However, whether this secretion is equally poisonous in all the species provided with that axillary sac, or whether it has poisonous qualities at all, is a question which can be decided by experiments only made with the living fishes. CHAPTER XV. DISTRIBUTION OF FISHES IN TIME. 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 Leptocardii and Cyclostomes, the lowest of the vertebrate series, must have preceded the other sub-classes, is an idea which has been held by many Zoologists: and as the horny teeth of the Cyclostomes are the only parts of their body which under favourable circumstances might have been preserved, Palæontologists have ever been searching for this evidence. [Illustration: Fig. 101. Right dental plate of Myxine affinis.]