514 FISHES. 



tropical freshwater fishes, they are able to survive droughts, 

 living in semi-fluid mud, or lying in a torpid state below the 

 hard-baked crusts of the bottom of a tank from which every 

 every drop of water has disappeared. Eespiration is probably 

 entirely suspended during the state of torpidity, but whilst 

 the mud is still soft enough to allow them to come to the 

 surface, they rise at intervals to take in a quantity of air, by 

 means of which their blood is oxygenised. This habit has 

 been observed in some species to continue also to the period 



Fig. 234. — Opliiocei^lialus striatus, India. 



of the year in which the fish lives in normal water, and indi- 

 viduals which are kept in a basin and prevented from coming 

 to the surface and renewing the air for respiratory purposes, 

 are suffocated. The particular manner in which the accessory 

 branchial cavity participates in respiratory functions is not 

 known. It is a simple cavity, without an accessory branchial 

 organ, the opening of which is partly closed hy a fold of the 

 mucous membrane. 



Sixteenth Division — Acanthopterygii Labyeinthi- 

 branchii. 



Body compressed, oblong or elevated, with scales of moderate 

 size. A superhrancliial organ in a cavity accessory to that of 

 the gills. 



First Family — Labyrinthici. 



Dorsal and anal spines 'present, hut in variable numhers ; 

 ventrals thoracic. Lateral line absent, or more or less distinctly 

 interrupted. Gill-opening rather narrow, the gill-membranes 



