432 ADAPTATIONS TO MEDIA AND SUBSTRATES 



The most nearly terrestrial of fishes are certain gobies and blennies. 

 Among the gobies, ecologically speaking, almost anything may happen. 

 They present a wide variety of bizarre adaptations and hold a number of 

 records of various sorts. Some of them, less than half an inch long when 

 fully grown, are the smallest of all vertebrates. Others, Periophthalmus, 

 Boleophthalmus, et al, actually prefer to spend most of their time out of 

 water on a mud-flat exposed at low tide. Most gobies have the pelvic fins 

 converted into an adhesive disc, and some of them cling with this to 

 wave-dashed rocks or to the sides of burrows, like the blind Typhlogobius 

 mentioned above (p. 388). It is not surprising that some surf -tossed gobies 

 have sought still greater security by getting out of the water altogether. 

 A still larger number of the blennies inhabit rocky places between the 

 tide-marks. The blennies lack the suctorial attachment organ, but their 

 amphibious members equal the amphibious gobies in pertness, fearless- 

 ness, and lizard-like agility. In keeping with these qualities they have 

 speedier accommodation than any other fishes. 



Periophthalmus and its relative Boleophthalmus, among the mud-skip- 

 pers of the coasts of Asia, West Africa, and Polynesia, have had a good 

 deal of attention. Their eyes are set in high turrets (Fig. 145a) and are 

 practically on universal joints, compensating thus for the lack of a neck 

 which becomes quite a handicap on land. They rotate under secondary 

 spectacles which appear to be their only protection against desiccation. 

 When deeply retracted into the head for mechanical protection, the eyes 

 are covered by puckered skin-folds somewhat as in the rays, anglers, and 

 turret-eyed flatfishes, which similarly have the body often in one medium 

 (sand) while the eyes are out in another (water). When the eye of a 

 mud-skipper is turned downward for horizontal vision like that of other 

 fishes, the skin forms a sort of lower 'lid'. This lid is only temporary, 

 and is abolished when the eye is elevated. The manner in which the infer- 

 ior rectus and inferior oblique muscles are crossed, in the mud-skippers, 

 makes of them a sort of cat's-cradle which raises the eye in its conning 

 tower. There is thus no need of a special levator bulbi muscle such as the 

 frog possesses. 



In an average adult of Periophthalmus koelreuteri, the eyeball is 4.0 

 mm. in diameter with a very large (3.8 mm.) and strongly curved cornea 

 (Fig. 145b). The lens is slightly flattened, its equatorial diameter being 

 1.14 times the axial. The static optics of the eye are thus those of a land 

 animal: Periophthalmus, when in the air, appears to be emmetropic or 

 even slightly hypermetropic — but the fish is then actually accommodating 



