In Acanthurus (Monoceros, Naseus), the unicorn-fish and its relatives, the ventral fins are reduced, having but three soft rays, the caudal spines are very large, blunt, immovable, one placed in front of the other. In most of the species of Acanthurus a long, bony horn grows forward from the cranium above the eye. This is wanting in the young and has various degrees of development in the different species, in some of which it is wholly wanting. The species of Acanthurus reach a large size, and in some the caudal spines are bright scarlet, in others blue. Acanthurus unicornis, the unicorn-fish, is the commonest species and the one with the longest horn. It is abundant in Japan, in Hawaii, and in the East Indies.
Axinurus thynnoides of the East Indies has a long, slim body, with slender tail like a mackerel.
Suborder Amphacanthi, the Siganidæ.—The Amphacanthi (ἄμφϊ, everywhere; ἄκανθα, spine) are spiny-rayed fishes certainly related to the Teuthididæ, but differing from all other fishes in having the last ray of the ventrals spinous as well as the first, the formula being I. 4, I. The anal fin has also six or seven spines; and the maxillary is soldered to the premaxillary. The skeleton is essentially like that of the Acanthuridæ.
The single family, Siganidæ, contains fishes of moderate size, valued as food, and abounding about rocks in shallow water from the Red Sea to Tahiti. The coloration is rather plain olive or brown, sometimes with white spots, sometimes with bluish lines. The species are very much alike and all belong to the single genus Siganus. One species, Siganus fuscescens, dusky with small, pale dots, is a common food-fish of Japan. Others, as Siganus oramin and Siganus vermiculatus, occur in India, and Siganus punctatus, known as lo, abounds about the coral reefs of Samoa. Siganus vulpinus differs from the others in the elongate snout.
A fossil genus, Archoteuthis (glaronensis), is found in the Tertiary of Glarus. It differs from Siganus in the deeper body and in the presence of six instead of seven spines in the anal fin.
The real relationship of the Siganidæ is still uncertain, but the family is probably most nearly allied to the Acanthuridæ, with which the species were first combined by Linnæus, who included both in his genus Teuthis. In the structure of the vertical fins the Siganidæ resemble the extinct genus Pygæus.
CHAPTER XXIV
SERIES PLECTOGNATHI
The Plectognaths.—Derived directly from the Acanthuridæ, from which they differ by progressive steps of degeneration, are the three suborders of Sclerodermi, Ostracodermi, and Gymnodontes, forming together the series or suborder of Plectognathi. As the members of this group differ from one another more widely than the highest or most generalized forms differ from the Acanthuridæ, we do not regard it as a distinct order. The forms included in it differ from the Acanthuridæ much as the swordfishes differ from ordinary mackerel. The Plectognathi (πλεϡτός, woven together; γνάθος, jaw) agree in the union of the maxillary and premaxillary, in the union of the post-temporal with the skull, in the great reduction of the gill-opening, and in the elongation of the pelvic bones. All these characters in less degree are shown in the Squamipinnes. We have also the reduction and final entire loss of ventral fins, the reduction and loss of the spinous dorsal, the compression and final partial or total fusion of the teeth of the upper jaw, the specialization of the scales, which change from bony scutes into a solid coat of mail on the one hand, and on the other are reduced to thorns or prickles and are finally altogether lost. The number of vertebræ is also progressively reduced until in the extreme forms the caudal fin seems attached to the head, the body being apparently wanting. Throughout the group poisonous alkaloids are developed in the flesh. These may produce the violent disease known as ciguatera, directly attacking the nervous system. See p. 182, vol. I.
The three suborders of plectognathous are easily recognized by external characters. In the Sclerodermi (σκλερός, hard; δέρμα, skin) the spinous dorsal is present and the body is more or less distinctly scaly. The teeth are separate and incisor-like and the form is compressed. In the Ostracodermi (ὀστράκος, a box; δέρμα, skin) there is no spinous dorsal, the teeth are slender, and the body is inclosed in an immovable, bony box. In the Gymnodontes (γυμνός, naked; ὀδούς, tooth) the teeth are fused into a beak like that of a turtle, either continuous or divided by a median suture in each jaw, the spinous dorsal is lost, and the body is covered with thorns or prickles or else is naked.