In _Thalassophryne_, a genus of toad-fishes of tropical America, is found the most perfect system of poison organs known among fishes. The spinous armature of the opercle and the two spines of the first dorsal fin constitute the weapons. The details are known from the dissections of Dr. Günther. According to his[19] observations, the opercle in _Thalassophryne_ "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 at 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." [Illustration: FIG. 135.--Brown Tang, _Teuthis bahianus_ (Ranzani). Tortugas, Florida.] =The Lancet of the Surgeon-fish.=--Some fishes defend themselves by lashing their enemies with their tails. In the tangs, or surgeon-fishes (_Teuthis_), the tail is provided with a formidable weapon, a knife-like spine, with the sharp edge directed forward. This spine when not in use slips forward into a sheath. The fish, when alive, cannot be handled without danger of a severe cut. In the related genera, this lancet is very much more blunt and immovable, degenerating at last into the rough spines of _Balistapus_ or the hair-like prickles of _Monacanthus_. =Spines of the Sting-ray.=--In all the large group of sting-rays the tail is provided with one or more large, stiff, barbed spines, which are used with great force by the animal, and are capable of piercing the leathery skin of the sting-ray itself. There is no evidence that these spines bear any specific poison, but the ragged wounds they make are always dangerous and often end in gangrene. It is possible that the mucus on the surface of the spine acts as a poison on the lacerated tissues, rendering the wound something very different from a simple cut. [Illustration: FIG. 136.--Common Filefish, _Stephanolepis hispidus_ (Linnæus). Virginia.] =Protection Through Poisonous Flesh of Fishes.=--In certain groups of fishes a strange form of self-protection is acquired by the presence in the body of poisonous alkaloids, by means of which the enemies of the species are destroyed in the death of the individual devoured.