POISON GLANDS 177 



dorsal fin is almost precisely similar to the opercular spine- 

 There is no evidence as to how the poison is ejected into a 

 wound, and it can only be conjectured that it may be caused by 

 the pressure exerted on the gland when the spine is forcibly 

 thrust for some distance into the flesh; Certain it is that these 

 structures are capable of inflicting painful and troublesome 

 wounds when the Fish is incautiously handled and the skin 

 accidentally punctured, and no doubt they can be used with 

 great effect as offensive organs. 



A similar poison apparatus exists in certain species of Batra- 

 chidae, such as Thalassophryne reticulata} which is by no means 

 uncommon at Panama. This apparatus is formed by a spinous 

 outgrowth from the opercular bone and by the first two dorsal 

 spines. Instead, however, of having two grooves, the opercular 

 spine resembles the fang of a venomous snake, and is perforated 

 by a complete canal which is only open at the base and point of 

 the spine. A poison-sac at the base of the spine discharges its 

 contents into the canal. The nature of the glands which secrete 

 the poison has yet to be discovered, but it is probable either that 

 there are glands in connexion with the poison-sac, or that the 

 latter is lined by a glandular epithelium. The structure of the 

 dorsal spines is similar. In some species of the Scorpaenoid 

 genus Synancia ^ (e.g. S. verrucosa, from the Indian Ocean), the 

 terminal portions of the dorsal spines are deeply grooved on each 

 side, and at the origin of each groove there is a pear-shaped bag 

 containing a milky poison. The bag is prolonged into a duct which, 

 after traversing the groove, opens at the extremity of the spine. 



Many Siluridae are armed with powerful and often serrated 

 dorsal and pectoral spines which are certainly capable of inflict- 

 ing dangerous wounds, and not a few of them possess a sac-like 

 organ with an external opening in the axilla of the pectoral fin. 

 It is possible that the sac secretes a poison for anointing the 

 spine, but at present there is no evidence that such is the case, 

 or that the sac produces any poisonous secretion at all.^ 



Among the Elasmobranchs the Eagle-Eays {Aetuhatis),*' and 

 the Sting-Eays {Trygon), have barbed or serrated spines on the 

 tail, which inflict wounds far more severe than those caused by 



1 Giinther, Trans. Zool. Soc. vi. 1869, p. 437. 



2 Ibid., Study of Fishes, Edinburgh, 1880, p. 191. 



3 Ibid. p. 192. ^ Ibid, p- 190. 



VOL. VII N 



