VENOMS IN THE ANIMAL SERIES 305 
most curious genus among the fishes belonging to this family is 
Holacanthus, in which the preoperculum is provided with an 
enormous spine like that of the Weever. Holocanthus imperator 
(fig. 114) is met with fairly often in the Indian Ocean and Malay 
Archipelago. 
ROLL OP VE LUS 
aise DT CREER 11 EL 
Fig. 114.—Holacanthus imperator (Indian Ocean and Malay Archipelago), 
(After Savtschenko.) 
B.—TELEOSTEI.—PLECTOGNATHI. 
The Order Plectognathi (Family Gymnodontes) includes the 
genera Diodon, Tetrodon and Triodon, elobular fishes, in which 
the Jaw is transformed into a beak and furnished with a sharp 
dentary plate. Their cesophagus is dilated into a resonant. air- 
pouch. When removed from the water they swallow air and 
dilate the pouch, and the expulsion of this air is accompanied 
by a loud noise. 
Several species of Tetrodon are armed with spines, which pro- 
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