688 



FISHES. 



produce a sound, doubtless by the expulsion of air from the 

 oesophagus. Their vertebral column consists of a small number 

 of vertebrae, from 20 to 29, and their spinal chord is extremely 

 short. All these fishes have a bad reputation, and they are 

 never eaten ; indeed, some of them are highly poisonous, and 

 have caused long continued illness and death. Singularly, 

 the poisonous properties of these fishes vary much as regards 

 intensity, only certain individuals of a species, or individuals 

 from a certain locality, or caught at a certain time of the year, 

 being dangerous. Therefore it is probable that they acqmre 

 their poisonous quality from their food, which consists in 

 corals and hard -shelled MoUusks and Crustaceans. Their 

 sharp beaks, with broad masticating posterior surface, are 

 admirably adapted for breaking off branchlets of coral-stocks, 

 and for crushing hard substances. 



Tetrodon (including Xenopterus). — Both the upper and lower 



jaws are divided into two by a mesial suture. 



Extremely numerous in tropical and sub- 

 tropical zones, more than sixty species being 

 known. In some of the species the dermal 

 spines are extremely small, and may be absent 

 altogether. Many are highly ornamented with 

 spots or bands. A few species live in large 

 rivers — thus T. psittacus from Brazil ; T. 

 faliaka, a fish well known to travellers on the 



NUe, and likewise abundant in West African rivers ; T. fluvi- 



Pig. 311 

 of Tetrodon, 



■Jaws 



Fig. 312. — Tetrodon margaritatus. 



