564 ORDER PLECTOGNATHI. 



But very small vestiges of ribs are to be found. The 

 true ventrals are wanting. The intestinal canal is 

 ample, but without coeca ' ; and almost all these fishes 

 have a considerable natatory bladder. 



This order comprehends two very natural families, 

 characterized by the manner in which their jaws are 

 armed, the Gymnodontes and Sclerodermes. 



The first familv, or the 



Gymnodontes, 



Has, instead of apparent teeth, the jaws furnished 

 with an ivory substance, divided internally into 

 lamina?, the general appearance of which somewhat 

 resembles the bill of a parrot, and which is essen- 

 tially composed of true teeth, united together, and 

 succeeding one another in proportion as there are 

 any worn out by the effect of trituration. (See my 

 Lessons on Comp. Anat. torn. hi. p. 125.) Their 

 opercula are small ; their rays are five in number 

 on each side, and both are very much concealed. 

 They live on Crustacea and fuci ; their flesh is gene- 

 rally mucous and not much esteemed. Several of 

 them are considered poisonous, at least in certain 

 seasons. 



Two of their genera, Tetrodon and Diodoti, 

 commonly called balloon-fish and globe-fish, are 

 capable of inflating themselves like balloons, by 



1 Bloch erroneously attributes cceea to the genus diodon. 



