"In the smallest specimen the maxillary expansion extends beyond the orbit for a distance about equal to that which intervenes between the anterior margin of the orbit and the tip of the snout; in No. 2 it reaches to the posterior margin of the preoperculum; in No. 3 it ends level with the gill-opening; while in the largest individual it passes the origin of the pectoral and ventral fins.
"What can be the use of this long fold of skin and cartilage, which is not attached to the head except where it joins the mouth, and which, from its gradual development and ultimate large dimensions, must certainly serve some useful purpose?
"Do not understand that I mean that every part of a creature is of use to it in its present mode of life, for, as all naturalists know, there are in structural anatomy, just as in social life, cases of survival; remains of organs which were at some former time more developed, parallel in their nature to such survivals in costume as the two buttons on the back of a man's coat, once useful for the attachment of a sword-belt. But in this fish we have no case of survival, but one of unusual development; the family (Gobiidæ) to which it belongs presents no similar case, although its members have somewhat similar habits, and the conviction grows upon us, as we consider the subject, that the long jaws serve some useful purpose in the economy of the creature. In view of the half-terrestrial life led by this fish, I am inclined to suspect that the expansion of the upper jaw may serve for the retention of a small quantity of water, which, slowly trickling downward into the mouth and gills, keeps the latter moist when, from an unusually low tide or a dry season, the waters of its native creek fail, perhaps for several hours, to reach the holes in which the fishes dwell. It may be objected to this view that, were such an appendage necessary or even useful, other species of Gobiidæ, whose habits are similar, would show traces of a similar adaptation. This, however, by no means follows. Nature has many ways of working out the same end; and it must be remembered that every real species, when thoroughly known, differs somewhat in habits from its congeners, or at least from its family friends. To take an illustration from the mammalia. The chimpanzee and the spider-monkey are both quadrumanous and both arboreal, yet the end which is attained in the former by its more perfect hands is reached in the latter by its prehensile tail.
"Why may not the extremely long channel formed by the jaw of this rather abnormal member of the goby family be another mode of provision for the requirements of respiration?"
Of the Asiatic genera, Periophthalmus and Boleophthalmus are especially notable. In these mud-skippers the eyes are raised on a short stalk, the fins are strong, and the animal has the power of skipping along over the wet sands and mud, even skimming with great speed over the surface of the water. It chases its insect prey among rocks, leaves, and weeds, and out of the water is as agile as a lizard. Several species of these mud-skippers are known on the coasts of Asia and Polynesia, Periophthalmus barbarus and Boleophthalmus chinensis being the best known. Awaous crassilabris is the common oopu, or river goby, of the Hawaiian streams, and Lentipes stimpsoni is the mountain oopu, capable of clinging to the rocks in the rush of torrents. Paragobiodon echinocephalus is a short thick-set goby with very large head, found in crevices of coral reefs of Polynesia.
Fig. 420.—Pond-skipper, Boleophthalmus chinensis (Osbeck). Bay of Tokyo, Japan. (Eye-stalks sunken in preservation.)
Fig. 421.—Mud-skippy, Periophthalmus oarbarus (L.). Mouth of Vaisigono River, Apia, Samoa.
In numerous interesting species the first dorsal fin is wanting or much reduced. The crystal goby, Crystallogobius nilssoni, of Europe is one of this type, with the body translucent. Equally translucent is the little Japanese shiro-uwo, or whitefish, Leucopsarion petersi. Mistichthys luzonius of the Philippine Islands, another diaphanous goby, is said to be the smallest of all vertebrates, being mature at half an inch in length. This minute fish is so very abundant as to become an important article of food in Luzon. The rank of "smallest-known vertebrate" has been claimed in turn for the lancelet (Asymmetron lucayanum), the top minnow, Heterandria formosa, and the dwarf sunfish (Elassoma zonatum). Mistichthys luzonius is smaller than any of these, but the diminutive gobies, called Eviota, found in interstices of coral rocks are equally small, and there are several brilliant but minute forms in the reefs of Samoa. The snake-like Eutæniichthys gilli of Japanese rivers is scarcely larger, though over an inch long. Typhlogobius californiensis, "the blindfish of Point Loma," is a small goby, colorless and blind, found clinging in dark crevices of rock about Point Loma and Dead Man's Island in southern California.