162 NATURAL HISTORY SOCIETY OF DUBLIN. 



which, in its common condition, is also filled with air ; but the animal 

 can, at will, press the air out of it, when the ridge collapses into a 

 membranous fold, the bladder remaining distended. On the lower side 

 of the bladder are the organs of nutrition, which consist of suckers and 

 prehensile filaments ; the former arise either singly from the bladder, 

 or many spring together from a common stem ; the prehensile filaments 

 consist of rounded filaments, covered throughout their whole length on 

 one side with a series of reniform acetabula, and on the other side 

 supported by a narrow membrane, which accompanies them from the 

 root to the point. At the root of each prehensile filament, of which 

 there are many of different sizes on a single animal, is a long pointed 

 receptacle of fluid, attracted throughout almost its whole length to the 

 filaments, and only free at its apex. The acetabula of the prehensile 

 filaments appear to be the organs which secrete the mucus which pro- 

 duces the irritation of the human skin, and by which animals which 

 are seized are at once paralyzed." 



One opinion as to the cause the of pain inflicted by these Acalephae is, 

 that the mucus from the tentacles, which is undoubtedly the vehicle of 

 the irritation they cause, is possessed of some poisonous property ; another, 

 and the more received is, that the irritation is mechanical, due to the 

 rupture of numerous cnidae, or thread-cells, which cells, when ruptured, 

 set free numerous fine barbed filaments, capable of penetrating the 

 pores of the skin. 



The cnidce, or thread-cells, are ruptured under pressure or irritation, 

 and the barbed filament or thread contained in the cell is brought in 

 contact with the offender. These thread- cells are found unusually 

 large in the Physalia, where they are spherical in figure, and attain a 

 diameter of '003 of an inch.* 



There are, moreover, reasons other than anatomical, for regarding 

 the irritation of the skin from contact with these threads to be of a 

 mechanical rather than of a poisonous or chemical nature. It has been 

 shown by Kletzinsky that levigated asbestos rubbed upon the skin 

 causes a rash, analogous to that produced by a nettle-sting, which is 

 closely similar to that produced by the marine Acalephae ; and in the 

 case of the nettle-sting, it has not been proved that the irritation 

 is due to the assumed presence of formic acid in the leaves of this 

 plant, while many think it more likely to be owing to the number of 

 fine hairs from the leaf which enter the skin. Many setaceous larvae, 

 if handled, inflict severe so-called stings, followed by a rash and great 

 irritation ; the larvae of the admiral butterfly, of the nettle tortoise- 

 shell butterfly, of the gold-tailed moth, and of the procession-moth, 

 have, in many instances, produced severe irritation of the skin, similar 

 to that resulting from the hairs of the pods of Dolichos pruriens. In 

 the case of some larvae, as Papilio urticce, this stinging power has been 

 illogically attributed to their feeding upon the leaves of nettles. Other 



* " Manual of Coelenterata," Greene. 



