Adaptations of Fishes 



i8i 



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 im- 

 mediately 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." 



The Lancet of the Surgeon-fish. — Some fishes defend themselves 

 by lashing their enemies with their tails. In the tangs, or surgeon- 

 fishes {Tenthis), the tail is provided with a formidable weapon, 



Fig. 1.3.5.— Brown Tang, Teuthi.'< bahianus (Ranzani). Tortugas, Florida. 



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. 



