AMPniBIOtJS FISHES. 189 



absorb oxygen from a current of water which enters by the 

 mouth, bathes the gills, and passes out again behind the oper- 

 culum through the gill-opening. Hence fishes would appear to 

 be confined exclusively to a life in the water. Nevertheless 

 there are a few forms which are able to breathe out of water, 

 and which in fact even pass a great part of their lives out of 

 water. Such are the two genera, both belonging to the 

 Gobiidce, Periophthalmus and Boleophthalmus ; these skip along 

 close to the water-line on the sea-shore, where they hunt for 

 Molluscs (Onchidium) and Insects. In their branchial cavity, 

 like all fishes, they have true gills ; but these, though not dif- 

 fering widely from those of other fishes living constantly in the 

 water, are far from filling up the cavity, which is rather large ; 

 and this seems to contain not merely water, but air as well. 

 In other fishes that occasionally visit land, the branchial cavity 

 on each side is prolonged, and penetrates upwards far into the 

 head, while its mucous membrane sometimes forms labyrinthine 

 folds of highly complicated structure (see fig. 47) ; hence their sur- 

 face is often much more extensive than that of the true gills. 

 Formerly the species which possess this accessory labyrinthine 

 organ in the gill-cavity were classed in one family of Labyrin- 

 thici, for it was thought that their internal affinities were 

 clearly denoted by the presence of this organ. Now, on the 

 contrary, they are distributed into several different families,'' 

 since it is considered as proved that their real genealogical 

 affinity is indicated by other characters, while the labyrinthine 

 organ must have originated independently in certain forms of 

 these different families, though with great similarity of struc- 

 ture and identical physiological functions. Formerly this func- 

 tion was explained by the hypothesis that water could be stored 

 in the cavity of the labyrinthine organ, which might be closed, 

 and that this water, being rich in oxygen, and unable to 

 escape from the gill-cavity, enabled the creature during its 

 excursions on land still to breathe in or through water. There 

 can be no doubt, since numerous observations exist on this 

 point, that they are capable of living for days out of water ; 

 many of them make long journeys over land, and some are even 

 said to be able, by means of the spines on their scales and gill- 



