An introduction to the study of fishes

by Albert C. L. G. Günther

Fig. 231.—Centriscus humerosus.

Of the four species the most generally known is C. scolopax, the “Trumpet-fish” or “Bellows-fish,” which rarely occurs on the south coast of England, is more common farther south, and reappears in Tasmania. The allied C. gracilis is one of the fishes common to the Mediterranean and Japanese Seas. The species figured, C. humerosus, occurs on the coast of South Australia, and is very scarce.

Amphisile.—Body elongate, strongly compressed, provided with a dorsal cuirass, which is formed by portions of the skeleton; the longitudinal axis of the tail is not in the same line with that of the trunk. Scales none. Teeth none. Two dorsal fins situated on the hindmost part of the back; ventral fins rudimentary, abdominal. Three or four branchiostegals.

The three species known of this genus are found in the tropical Indo-Pacific. Their body is so thin that it has the appearance of being artificially compressed between two sheets of paper; it is semi-transparent, especially in the region of the air-bladder. The structure of the vertebral column is extremely singular and unique among Acanthopterygians. The abdominal portion is more than four times as long as the caudal; nevertheless it is composed of only six vertebræ, whilst the latter consists of fourteen. The abdominal vertebræ are extremely slender, the third alone being nearly as long as the whole caudal portion; they have a slight ridge superiorly and inferiorly, and on each side; the whole portion lying in the uppermost concavity of the dorsal cuirass. The caudal vertebræ are extremely short, and the strength of their neural and hæmal spines is in proportion to their size. The dorsal cuirass is not a dermal production, but formed by modified parts of the endoskeleton; its composition, the number and condition of its single parts, and, finally, the first dorsal spine, which in A. punctulata is so singularly attached to it, favour this opinion. The plates, which occupy the vertebral line, would correspond to the neural spines, and the lateral plates on which the ribs are suspended to the parapophyses. Amphisile may be considered as a Chelonian form among fishes.

Fourteenth Division—Acanthopterygii Gobiesociformes.

No spinous dorsal; the soft and the anal short or of moderate length, situated on the tail; ventral fins subjugular, with an adhesive apparatus between them. Body naked.

These fishes are well characterised by their single dorsal fin, and by their adhesive ventral apparatus, which has only an external similarity to the organ observed in Cyclopterus and Liparis; its structure is typically different from it. Whilst in those genera the ventral fins occupy the centre of the disk forming its base, these fins are here widely apart from each other, as in Callionymus, forming only a portion of the periphery of the disk, which is completed by a cartilaginous expansion of the coracoid bones. The following description of its structure is taken from Sicyases sanguineus, but it is essentially the same in all the genera.

The whole disk is exceedingly large, subcircular, longer than broad, its length being one-third of the whole length of the fish. The central portion is formed merely by skin, which is separated from the pelvic or pubic bones by several layers of muscles. The peripheric portion is divided into an anterior and posterior part by a deep notch behind the ventrals. The anterior peripheric portion is formed by the four ventral rays, the membrane between them, and a broad fringe which extends anteriorly from one ventral to the other; this fringe is a fold of the skin, containing on each side the rudimentary ventral spine, but no cartilage. The posterior peripheric portion is suspended on each side from the coracoid, the upper bone of which is exceedingly broad, becoming a free movable plate behind the pectoral. A broad cartilage is firmly attached to it. The lower bone of the coracoid is of a triangular form, and supports a very broad fold of the skin, extending from one side to the other, and containing a cartilage which runs through the whole of that fold. Five processes of the cartilage are continued into the soft striated margin in which the disk terminates posteriorly. The surface of the disk is coated with thick epidermis, like the sole of the foot of higher animals. The epidermis is divided into many polygonal plates; there are no such plates between the roots of the ventral fins.

Not less unique is the structure of the bones which have some relation to this external adhesive apparatus. As exemplified by Chorismochismus dentex the coracoid is well developed, and, as usual, composed of two pieces, the upper of which is not suspended from the humerus, but fixed by a ligament to the hinder margin of the carpal bones. It is a broad lamella, dilated posteriorly into the cartilage, which is externally visible; the lower piece is narrower, and fixed to the extremity of the pubic bone of its side. The pubic bones are united by suture, and form together a heart-shaped disk, the point of which is produced backwards. The anterior portion of the disk is concave, with a bony longitudinal bridge and a feeble transverse ridge. The disk is fixed to the humeral bones by the convex portions of its anterior margin, whilst the convex portions of the lateral margins serve as base for the ventral fins. The latter are composed of one spine, which is transformed into a broad, thin, and curved plate, hidden below the skin, and apparently of four rays; but on closer examination we find that the hidden ray has a longitudinal groove anteriorly, in which another thinner ray lies concealed. This ray is quite free, and not joined to the pubic bone.