Suborder Hemibranchii.—Still another transitional group, the Hemibranchii, is composed of spiny-rayed fishes with abdominal ventrals. In this suborder there are other points of divergence, though none of high importance. In these fishes the bones of the shoulder-girdle are somewhat distorted, the supraclavicle reduced or wanting, and the gill structures somewhat degenerate. The presence of bones called interclavicles or infraclavicles, below and behind the clavicle, has been supposed to characterize the order of Hemibranchii. But this character has very slight importance. In two families, Macrorhamphosidæ and Centriscidæ, the interclavicles are absent altogether. In the Fistulariidæ they are very large. According to the studies of Mr. Edwin C. Starks, the bone in question is not a true infraclavicle. It is not identical with the infraclavicle of the Ganoids, but it is only a backward extension of the hypocoracoid, there being no suture between the two bones. In those species which have bony plates instead of scales, this bone has a deposit of bony substance or ganoid enamel at the surface. This gives it an apparent prominence as compared with other bones of the skeleton, but it has no great taxonomic importance. Dr. Hay unites the suborders Hemibranchii, Lophobranchii, and Hypostomides to form the order Phthinobranchii (φθινάς, waning; βράγχος, gill), characterized by the reduction of the gill-arches. These forms are really nearly related, but their affinities with the Percesoces are so close that it may not be necessary to form a distinct order of the combined group. Boulenger unites the Hemibranchii with Lampris to form a group, Catosteomi, characterized by the development of infraclavicles; but we cannot see that Lampris bears any affinity to the sticklebacks, or that the presence of infraclavicle has any high significance, nor is it the supposed infraclavicle of Lampris homologous with that of the Hemibranchii. The dorsal fin in the Hemibranchii has more or less developed spines; spines are also present in the ventral fins. The lower pharyngeals are separated; there is no air-duct. The mouth is small and the bones of the snout are often much produced. The preopercle and symplectic are distinct. The group is doubtless derived from some transitional spiny-rayed type allied to the Percesoces. The Lophobranchs, another supposed order, represent simply a still further phase of degradation of gills and ventral fins. Dr. Gill separates these two groups as distinct orders and places them, as aberrant offshoots, near the end of his series of bony fishes. We prefer to leave them with the other transitional forms, not regarding their traits of divergence as of any great importance in the systematic arrangement of families.
The Sticklebacks: Gasterosteidæ.—The sticklebacks (Gasterosteidæ) are small, scaleless fishes, closely related to the Fistulariidæ so far as anatomy is concerned, but with very different appearance and habits. The body often mailed, the dorsal is preceded by free spines and the ventrals are each reduced to a sharp spine with a rudimentary ray. The jaws are short, bristling with sharp teeth, and these little creatures are among the most active, voracious, and persistent of all fishes. They attack the fins of larger fishes, biting off pieces, and at the same time they devour the eggs of all species accessible to them. In almost all fresh and brackish waters of the north temperate zone these little fishes abound. "It is scarcely to be conceived," Dr. Günther observes, "what damage these little fishes do, and how greatly detrimental they are to the increase of all the fishes among which they live, for it is with the utmost industry, sagacity, and greediness that they seek out and destroy all the young fry that come their way."
The sticklebacks inhabit brackish and fresh waters of the northern hemisphere, species essentially alike being found throughout northern Europe, Asia, and America. The same species is subject to great variation. The degree of development of spines and bony plates is greatest in individuals living in the sea and least in clear streams of the interior. Each of the mailed species has its series of half-mailed or even naked varieties found in the fresh waters. This is true in Europe, New England, California, and Japan. The farther the individuals are from the sea, the less perfect is their armature. Thus, Gasterosteus cataphractus, which in the sea has a full armature of bony plates on the side, about 30 in number, will have in river mouths from 6 to 20 plates and in strictly fresh water only 2 or 3 or even none at all.
The sticklebacks have been noted for their nest-building habits. The male performs this operation, and he is provided with a special gland for secretion of the necessary cement. Dr. Gill quotes from Dr. John A. Ryder an account of this process. The secretory gland is a "large vesicle filled with a clear secretion which coagulates into threads upon contact with water. It appears to open directly in front of the vent. As soon as it is ruptured, it loses its transparency, and whatever secretion escapes becomes whitish after being in contact with water for a short time. This has the same tough, elastic qualities as when spun by the animal itself, and is also composed of numerous fibers, as when a portion is taken that has been recently spun upon the nest. Thus provided, when the nuptial season has arrived the male stickleback prepares to build his nest, wherein his mate may deposit her eggs. How this nest is built, and the subsequent proceedings of the sticklebacks, have been told us in a graphic manner by Mr. John K. Lord, from observations on Gasterosteus cataphractus on Vancouver Island, although the source of his secretion was misunderstood:
"The site is generally amongst the stems of aquatic plants, where the water always flows but not too swiftly. He first begins by carrying small bits of green material which he nips off the stalks and tugs from out the bottom and sides of the bank; these he attaches by some glutinous material, that he clearly has the power of secreting, to the different stems destined as pillars for his building. During this operation he swims against the work already done, splashes about, and seems to test its durability and strength; rubs himself against the tiny kind of platform, scrapes the slimy mucus from his sides to mix with and act as mortar for his vegetable bricks. Then he thrusts his nose into the sand at the bottom, and, bringing a mouthful, scatters it over the foundation; this is repeated until enough has been thrown on to weight the slender fabric down and give it substance and stability. Then more twists, turns, and splashings to test the firm adherence of all the materials that are intended to constitute the foundation of the house that has yet to be erected on it. The nest, or nursery, when completed is a hollow, somewhat rounded, barrel-shaped structure worked together much in the same way as the platform fastened to the water-plants; the whole firmly glued together by the viscous secretion scraped from off the body. The inside is made as smooth as possible by a kind of plastering system; the little architect continually goes in, then, turning round and round, works the mucus from his body on to the inner sides of the nest, where it hardens like tough varnish. There are two apertures, smooth and symmetrical as the hole leading into a wren's nest, and not unlike it.
"All this laborious work is done entirely by the male fish, and when completed he goes a-wooing. Watch him as he swims towards a group of the fair sex enjoying themselves amidst the water-plants arrayed in his best and brightest livery, all smiles and amiability; steadily and in the most approved style of stickleback love-making this young and wealthy bachelor approaches the object of his affections, most likely tells her all about his house and its comforts, hints delicately at his readiness and ability to defend her children against every enemy, vows unfailing fidelity, and in lover fashion promises as much in a few minutes as would take a lifetime to fulfill. Of course she listens to his suit; personal beauty, indomitable courage, backed by the substantial recommendations of a house ready built and fitted for immediate occupation, are gifts not to be lightly regarded.
"Throwing herself on her side the captive lady shows her appreciation, and by sundry queer contortions declares herself his true and devoted spouse. Then the twain return to the nest, into which the female at once betakes herself and therein deposits her eggs, emerging, when the operation is completed, by the opposite hole. During the time she is in the nest (about six minutes) the male swims round and round, butts and rubs his nose against it, and altogether appears to be in a state of defiant excitement. On the female leaving, he immediately enters, deposits the milt on the eggs, taking his departure through the back door. So far his conduct is strictly pure; but I am afraid morality in stickleback society is of rather a lax order. No sooner has this lady, his first love, taken her departure, than he at once seeks another, introduces her as he did the first, and so on, wife after wife, until the nest is filled with eggs, layer upon layer, milt being carefully deposited betwixt each stratum of ova. As it is necessary there should be two holes, by which ingress and egress can be readily accomplished, so it is equally essential in another point of view. To fertilize fish-eggs, running water is the first necessity; and, as the holes are invariably placed in the direction of the current, a steady stream of water is thus directed over them."
To the genus Gasterosteus the largest species belong, those having three dorsal spines, and the body typically fully covered with bony plates. Gasterosteus aculeatus inhabits both shores of the Atlantic and the scarcely different Gasterosteus cataphractus swarms in the inlets from southern California to Alaska, Siberia, and northern Japan. Half-naked forms have been called by various names and one entirely naked in streams of southern California is named Gasterosteus williamsoni. Its traits are, however, clearly related to its life in fresh waters.
In Pygosteus pungitius, a type of almost equally wide range, there are nine or ten dorsal spines and the body is more slender. All kinds of waters of the north on both continents may yield this species or its allies and variations, mailed or naked. The naked, Apeltes quadracus, is found in the sea only, along the New England coast.