Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/680

662 662 ICHTHYOLOGY [GROWTH AND movably joined, and special muscles serve to regulate tlieir movements. Sometimes they are armed with hook-like osseous excrescences (Selache). They are irregularly convo luted longitudinally, and, when closely pressed to each other, form a canal open at their extremity. A gland, which discharges a secretion abundantly during the season of propagation, is situated at the base of the canal, and opens into it. It is still doubtful whether the generally adopted opinion that their function con sists in holding the female during copulation is correct, or whether they are not rather an intromittent organ, the canal of which conducts, not only the secretion of their proper gland, but also the impregnating fluid. The ova of the oviparous Chondropterygians are large, and few in number ; they are successively impregnated, and the impregnation must take place before they are invested with a tough leathery envelope, which would be impenetrable to the semen, that is, before they nter the uterus ; therefore, copulation must take place in all these fishes. The form of the egg-shell differs in the various genera; usually (fig. 57) they are flattened and quadrangular, with each of the four corners produced, and fre quently prolonged into, lengthy J 1 * i Fia - ST. Egg of a ScyUium from nlamentS Which Serve for the Magellan s Straits (? Sc. chilense), attachment of the ova to other Iiatural size - fixed objects. In Notidanus the surfaces are crossed by numerous ridges. In Cestracion the egg is pyriform, with two broad ridges or plates wound edgewise round it, the two ridges forming five spires. The eggs of Callorhynchus have received a protective resemblance to a broad-leaved f ucus, forming a long depressed ellipse, with a plicated and fringed margin. GROWTH AND VARIATION OF FISHES. Changes of form normally accompanying growth (after absorption of the vitelline sac) are observed in all fishes, but in the majority these affect only the proportional size of the various parts of the body. Relatively to the size of the head, the eyes in young fishes are always larger than in the adult; and again, the head is relatively larger than the body. Changes amounting to metamorphosis have been hitherto observed in Petromyzon only. In the larval condition (Ammocoetes) the head is very small, and the toothless buccal cavity is surrounded by a semicircular upper lip. The eyes are extremely small, hidden in a shallow groove ; and the vertical fins form a continuous fringe. In the course of three or four years the teeth are developed, and the mouth changes into a perfect suctorial organ ; the eyes grow ; and the dorsal fin is separated into two divisions. In Malacopterygians and Anacanths the em bryonal fringe from which the vertical fins are developed is much longer persistent than in Acanthopterygians. A meta morphosis relating to the respiratory organs, as in Eatra- chians, is indicated in the class of fishes by the external gills with which foetal Plagiostomes and the young of some Ganoids, viz., the Protopterus and Polypterus, are provided. One of the most extraordinary changes by which, during growth, the form and position of several important organs are affected, occurs in flat-fishes (P.leuronectidce) ; their young are symmetrically formed, with a symmetrical mouth, and with one eye on each side, and therefore keep their body in a vertical position when swimming. As they grow they live more on the bottom, and their body, during rest, assumes a horizontal position ; in consequence, the eye of the lower side moves towards the upper, which alone is coloured ; and in many genera the mouth is twisted in the opposite direction, so that the bones, muscles, and teeth are much more developed on the blind side than on the coloured. In a great number of other Teleostei certain bones of the head show a very different form in the young state. Ossification proceeds in those bones in the direction of lines or radii which project in the form of spines or pro cesses ; as the interspaces between these processes are filled with bone, the processes disappear entirely, or at least project much less in the older than in the younger indi viduals. The young of some fishes may be armed with a long powerful preeopercular or scapular spine, or may show a serrature of which nothing remains in the adult fish except some ridges or radiating lines. These processes seem to serve as weapons of defence during a period in the life of the fish in which it needs them most. In not a few in- FIG. 58. Tholichthys osseus (six times the natural size). stances a portion of this armature is so much developed that the disappearance of its most projecting parts with the growth of the fish is not only due to its being surrounded by other bone, but partially, at least, caused by absorption. The Carangidce, Cyttidce, Squamipinnes, Xiphiidw, offer instances of such remarkable changes. A fish described as Tholichthys osseus (fig. 58) is probably the young of a Cyttoid, the suprascapula, humerus, and prseoperculum forming enormously enlarged plates. In another fish (fig. 59) these bones appear still enlarged, and the frontals de velop a remarkably long and curved horn above the orbit. In the Tholickthys-stSigQ of Pomacantlius (specimens 10 millimetres long), the frontal bone is prolonged into a straight lancet-shaped process, nearly half as long as the body ; the suprascapular and prseopercular processes cover and hide the dorsal and ventral fins. The plates attached to the shoulder-girdle re main persistent until the young fish has assumed the form of the adult ; thus they are still visible in young Chceto- don citrinellus, 30 millimetres long, in which the specific characters are already fully developed (fig. 60). The sword- fishes with ventral fins (Histiophoriis) belong to the Tele- osteans of the largest size ; in young individuals, 9 milli metres long (fig. 61), both jaws are produced, and armed FIG. 59. Tholichthys-stage of Heniochus (?).