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.

Eucalia inconstans is the stickleback of the clear brook from New York to Indiana and Minnesota. The male is jet black in spring with the sheen of burnished copper and he is intensely active in his work of protecting the eggs of his own species and destroying the eggs and fry of others. Spinachia spinachia is a large sea stickleback of Europe with many dorsal spines.

No fossil Gasterosteidæ are recorded, and the family, while the least specialized in most regards, is certainly not the most primitive of the suborder.

The Aulorhynchidæ.—Closely related to the sticklebacks is the small family of Aulorhynchidæ, with four soft rays in the ventral fins. Aulorhynchus, like Spinachia, has many dorsal spines and an elongate snout approaching that of a trumpet-fish. Aulorhynchus flavidus lives on the coast of California and Aulichthys japonicus in Japan. The extinct family of Protosyngnathidæ is near Aulorhynchus, with the snout tubular, the ribs free, not anchylosed as in Aulorhynchus, and with the first vertebræ fused, forming one large one as in Aulostomus. Protosyngnathus sumatrensis occurs in Sumatra. Protaulopsis bolcensis of the Eocene of Italy has the ventral fins farther back, and is probably more primitive than the sticklebacks.

Cornet-fishes: Fistulariidæ.—Closely related to the sticklebacks so far as structure is concerned is a family of very different habit, the cornet-fishes, or cornetas (Fistulariidæ). In these fishes the body is very long and slender, like that of a garfish. The snout is produced into a very long tube, which bears the short jaws at the end. The teeth are very small. There are no scales, but bony plates are sunk in the skin. The ventrals are abdominal, each with a spine and four rays. The four anterior vertebræ are very much elongate. There are no spines in the dorsal and the back-bone extends through the forked caudal, ending in a long filament. The cornet-fishes are dull red or dull green in color. They reach a length of two or three feet, and the four or five known species are widely distributed through the warm seas, where they swim in shallow water near the surface. Fistularia tabaccaria, the tobacco-pipe fish, is common in the West Indies, Fistularia petimba, F. serrata, and others in the Pacific. A fossil cornet-fish of very small size, Fistularia longirostris, is known from the Eocene of Monte Bolca, near Verona. Fistularia kœnigi is recorded from the Oligocene of Glarus.

The Trumpet-fishes: Aulostomidæ.—The Aulostomidæ, or trumpet-fishes are in structure entirely similar to the Fistulariidæ, but the body is band-shaped, compressed, and scaly, the long snout bearing the feeble jaws at the end. There are numerous dorsal spines and no filament on the tail. Aulostomus chinensis (maculatus) is common in the West Indies, Aulostomus valentini abounds in Polynesia and Asia, where it is a food-fish of moderate importance. A species of Aulostomus (bolcensis) is found in the Italian Eocene. Allied to it is the extinct family Urosphenidæ, scaleless, but otherwise similar. Urosphen dubia occurs in the Eocene at Monte Bolca. Urosphen is perhaps the most primitive genus of the whole suborder of Hemibranchii.

The Snipefishes: Macrorhamphosidæ.—Very remarkable fishes are the snipefishes, or Macrorhamphosidæ. In these forms the snout is still tubular, with the short jaws at the end. The body is short and deep, partly covered with bony plates. The dorsal has a very long serrated spine, besides several shorter ones, and the ventral fins have one spine and five rays.

The snipefish, or woodcock-fish, Macrorhamphosus scolopax, is rather common on the coasts of Europe, and a very similar species (M. sagifue) occurs in Japan. The Rhamphosidæ, represented by Rhamphosus, an extinct genus with the ventrals further forward, are found in the Eocene rocks of Monte Bolca. Rhamphosus vastrum has minute scales, short dorsal, and the snout greatly attenuate.

The Shrimp-fishes: Centriscidæ.—One of the most extraordinary types of fishes is the small family of Centriscidæ, found in the East Indies. The back is covered by a transparent bony cuirass which extends far beyond the short tail, on which the two dorsal fins are crowded. Anteriorly this cuirass is composed of plates which are soldered to the ribs. The small toothless mouth is at the end of a long snout.

These little fishes with the transparent carapace look very much like shrimps. Centriscus scutatus (Amphisile) with the terminal spine fixed is found in the East Indies, and Æoliscus strigatus with the terminal spine movable is found in southern Japan and southwards.

A fossil species, Æoliscus heinrichi, is found in the Oligocene of various parts of Europe, and Centriscus longirostris occurs in the Eocene of Monte Bolca.

In the Centriscidæ and Macrorhamphosidæ the expansions of the hypocoracoid called infraclavicles are not developed.

The Lophobranchs.—The suborder Lophobranchii (λοφός, tuft; βραγχός, gill) is certainly an offshoot from the Hemibranchii and belongs likewise among the forms transitional from soft to spiny-rayed fishes. At the same time it is a degenerate group, and in its modifications it turns directly away from the general line of specialization.

The chief characters are found in the reduction of the gills to small lobate tufts attached to rudimentary gill-arches. The so-called infraclavicles are present, as in most of the Hemibranchii. Bony plates united to form rings take the place of scales. The long tubular snout bears the short toothless jaws at the end. The preopercle is absent, and the ventrals are seven-rayed or wanting. The species known as pipefishes and sea-horses are all very small and none have any economic value. They are numerous in all warm seas, mostly living in shallow bays among seaweed and eel-grass. The muscular system is little developed and all the species have the curious habit of carrying the eggs until hatched in a pouch of skin under the belly or tail; this structure is usually found in the male.

The Solenostomidæ.—The Solenostomidæ of the East Indies are the most primitive of these fishes. They have the body rather short and provided with spinous dorsal, and ventral fins. The pretty species are occasionally swept northward to Japan in the Black Current. Solenostomus cyanopterus is a characteristic species. Solenorhynchus elegans, now extinct (with the trunk more elongate), preceded Solenostomus in the Eocene of Monte Bolca.

The Pipefishes: Syngnathidæ.—The Syngnathidæ are very long and slender fishes, with neither spinous dorsal, nor ventral fins, the body covered by bony rings. Of the pipefish, Syngnathus, there are very many species on all northern coasts. Syngnathus acus is common in Europe, Syngnathus fuscum along the New England coast, Syngnathus californiense in California, and Syngnathus schlegeli in Japan. Numerous other species of Syngnathus and other genera are found further south in the same regions. Corythroichthys is characteristic of coral reefs and Microphis of the streams of the islands of Polynesia. In general, the more northerly species have the greater number of vertebræ and of bony rings. Tiphle tiphle is a large pipefish of the Mediterranean. This species was preceded by Tiphle albyi (Siphonostoma) in the Miocene of Sicily. Other pipefishes, referred to as Syngnathus and Calamostoma, are found as fossils in Tertiary rocks.

The Sea-horses: Hippocampus.—Both fossil and recent forms constitute a direct line of connection from the pipefishes to the sea-horses. In the latter the head has the form of the head of a horse. It is bent at right angles to the body like the head of a knight at chess. There is no caudal fin, and the tail in typical species is coiled and can hardly be straightened out. Calamostoma of the Eocene, Gasterotokeus of Polynesia, and Acentronura of Japan are forms which connect the true sea-horses with the pipefish. Gasterotokeus has the long head and slender body of the pipefish, with the prehensile finless tail of a sea-horse. Most of the living species of the sea-horse belong to the genus Hippocampus. These little creatures have the egg-sac of the male under the abdomen. They range from two inches to a foot in length and some of the many species may be found in abundance in every warm sea. Some cling by the tails to floating seaweed and are swept to great distances; others cling to eel-grass and live very near the shore. The commonest European species is Hippocampus hippocampus. Most abundant on our Atlantic coast is Hippocampus hudsonius. Hippocampus coronatus is most common in Japan. The largest species are Hippocampus ingens of Lower California and Hippocampus kelloggi in Japan. Many species, especially of the smaller ones, have the spines of the bony plates of the body ending in fleshy flaps. These are sometimes so enlarged as to simulate leaves of seaweed, thus serving for the efficient protection of the species. These flaps are developed to an extreme degree in Phyllopteryx eques, a pipefish of the East Indies.

No fossil sea-horses are known.

The following account of the breeding-habits of our smallest sea-horse (Hippocampus zosteræ) was prepared by the writer for a book of children's stories:

"He was a little bit of a sea-horse and his name was Hippocampus. He was not more than an inch long, and he had a red stripe on the fin on his back, and his head was made of bone and it had a shape just like a horse's head, but he ran out to a point at his tail, and his head and his tail were all covered with bone. He lived in the Grand Lagoon at Pensacola in Florida, where the water is shallow and warm and there are lots of seaweeds. So he wound his tail around a stem of seaweed and hung with his head down, waiting to see what would happen next, and then he saw another little sea-horse hanging on another seaweed. And the other sea-horse put out a lot of little eggs, and the little eggs all lay on the bottom of the sea at the foot of the seaweed. So Hippocampus crawled down from the seaweed where he was and gathered up all those little eggs, and down on the under side of his tail where the skin is soft he made a long slit for a pocket, and then he stuffed all the eggs into this pocket and fastened it together and stuck it with some slime. So he had all the other sea-horse's eggs in his own pocket.

"Then he went up on the seawrack again and twisted his tail around it, and hung there with his head down to see what would happen next. The sun shone down on him, and by and by all the little eggs began to hatch out, and each one of the eggs was a little sea-pony, shaped just like a sea-horse. And when he hung there with his head down he could feel all the little sea-ponies squirming inside his pocket, and by and by they squirmed so much that they pushed the pocket open, and then every one crawled away from him, and he couldn't get them back, and so he went along with them and watched to see that nothing should hurt them. And by and by they hung themselves all up on the seaweeds, and they are hanging there yet. And so he crawled back to his own piece of seaweed and twisted his tail around it, and waited to see what would happen next. And what happened next was just the same thing over again."

Suborder Hypostomides, the Sea-moths: Pegasidæ.—The small suborder of Hypostomides (ὑπό, below; στόμα, mouth) consists of the family of Pegasidæ. These "sea-moths" are fantastic little fishes, probably allied to the sticklebacks, but wholly unique in form. The slender body is covered with bony plates, the gill-covers are reduced to a single plate. The small mouth underneath a long snout has no teeth. The preopercle and the symplectic are both wanting. The ventrals are abdominal, formed of two rays, and the very large pectoral fin is placed horizontally like a great wing.

The species, few in number, known as sea-moths and sea-dragons, rarely exceed four inches in length. They are found in the East Indies and drift with the currents northward to Japan. The genera are Pegasus, Parapegasus, and Zalises. The best-known species are Zalises draconis and Pegasus volitans.

No fossil species of Pegasidæ are known.