2o6 THE BIOLOGY OF THE SEA-SHORE 



to collapse or to become dried up and entangled with one 

 another. We have seen, however, that in fishes, Gasteropod 

 molluscs, and Crustaceans, the gills are quite effectively 

 protected. Now, it is precisely these groups that have 

 furnished truly amphibious forms, and we cannot doubt that 

 it is the protection given to the respiratory organs that has 

 rendered the partly terrestrial habits possible. 



Certain fishes which, however, are not represented in 

 Britain, have acquired the habit of leaving the water periodi- 

 cally and of disporting themselves in the air. Among these 

 are two species of goby : Periophthalmm (see Fig. 14) and 

 Boleophthalmus , which are said to skip along the water- 

 line on the sea-shore where they hunt for molluscs {Onchi- 

 dium) and insects (Semper, op. cit.). The gills in these 

 forms do not fill the branchial cavity, which is larger than 

 usual and apparently contains air as well as water. Several 

 other fishes belonging to unrelated families have their 

 gill- cavities extended a considerable distance upwards on 

 each side of the head, the mucous membrane of the extension 

 being sometimes folded in complicated fashion to form a 

 " labyrinthine organ." Formerly it was thought that the 

 function of these accessory organs was to hold a store of 

 water upon which the animals might draw during their 

 long excursions. It is now certain, however, that they 

 contain nothing but air and are really capable of respiring 

 atmospheric oxygen. Among the species which possess 

 this modification is the well-known " climbing-perch " 

 (Anabas scandens), which is popularly credited with the 

 power of mounting palm trees by the aid of the spines on 

 its scales and gill-covers, and is certainly capable of living 

 out of water notwithstanding that the accounts of its climbing 

 powers may be exaggerated. The amphibious habits of 

 these fishes and the relatively trivial amount of modification 

 that has been required to make them possible show that the 

 two processes of air-breathing and water-breathing are not 

 so dissimilar after all. This statement is further corrobo- 

 rated by the readiness with which fishes living in water 

 which is insufficiently aerated will come to the surface to 



