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The tail, or postanal region, is probably a secondary development-a prolongation of the hinder end of the body for motor purposes. Thisis indicated by the fact that it frequently develops late in ontogeny.

The vertebrate, in correlation perhaps with its extreme cephalization, develops from before backwards (except the alimentary canal, which develops more en bloc), there remaining at the hind end for a prolonged period a mass of undifferentiated embryonic tissue from the anterior side of which the definitive tissues are constantly being developed. After development has reached the level of the anus it still continues backwards and the tail region is formed, showing a continuation of the same tissues as in front, notochord, nerve cord, gut, myotomes. Of these the (postanal) gut soon undergoes atrophy. Fins.-The fins are extensions of the body surface which serve for propulsion. To give the necessary rigidity they are provided with special skeletal elements, while to give mobility they are provided with special muscles. These muscles, like the other voluntary muscles of the body, are derived from the primitive myotomes and are therefore segmental in origin. The fins are divisible into two main categories-the median or unpaired fins and the paired fins. The median fins are to be regarded as the more primitive. The fundamental structure of the vertebrate, with its median ea

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FIG. x.-Heterocercal Tail of Aclpeuser. a, Modified median 1 scales (“ fulcra ); 11, bony plates. skeletal axis and its great muscular mass divided into segments along each side of the body, indicates that its primitive method of movement was by waves of lateral fiexure, as seen in an Amphioxus, a cyclostome or an eel. The system of median tins consists in the first instance of a continuous fm-fold extending round the posterior end of the body»-as persists even in the adult in the existing Dipneusti. A continuous median fin-fold occurs also in various Teleosts (many deep-sea Teleosts, eels, ".": p 4 '-i



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From Cambridge Nalural Hislary, vol. vii., “ Fishes, &c., ” by permission of Messrs. Macmillan & Co., Ltd.

FIG. 2.-Cladoselache. (After Dean.) &c.), though the highly specialized features in other respects make it probable that we have here to do with a secondary return to a condition like the primitive one. In the process of segmentation of the originally continuous fm-fold we notice first of all a separation of and an increase in size of that portion of the fin which from its position at the tip of the tail region is in the most advantageous position for producing movements of the body. There is thus formed the caudal fin. In this region there is a greatly increased size of the fin-fold-both dorsally and ventrally. There is further developed a highly characteristic asymmetry. In the original symmetrical or prolocercal (=diphycercal) type of tail (as seen in a cyclostome, a Dipnoan and in most fish embryos) the skeletal axis of the body runs straight out to its tip-the tail fold being equally developed above and below the

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upwards as in the hclerocercal tail of sharks and sturgeons. The highest stage in this evolution of the caudal nn is seen in the Teleostean fishes, where the ventral tail-fold becomes developed to such an extent as to produce a secondarily symmetrical appearance (homocercal tail, fig. 4). ~ The sharks have been referred to as possessing heterocercal tails, but, though this is true of the majority, within the limits of the group all three types of tail-fin occur, from the protoeercal tail of the fossil Pleuracanthids and the living Chlamydoselachus to the highly rggvelcgped, practically homocercal tail of the ancient Cladoselachc g. 2

The praecaudal portion of the fin-fold on the dorsal side of the body becomes broken into numerous fmlets in living Crossopterygians, while in other fishes it disappears throughout part of its length, leaving only one, two or three enlarged portions the dorsal tins (fig. 4, d.f.). Similarly the praecaudal part of the fin-fold ventrally becomes reduced to a single anal fm (af), occasionally continued backwards by a series of finlets (Scambrldae). In. the sucker-fishes (Remora, Echenels) the anterior dorsal fin is metamorphosed into a sticker by which the creature attaches itself to larger fishes, turtles, &c. The paired fins-though more recent developments than the median-are yet of very great morphological interest, ! ll

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From “Challenger” Reporls Zool., published by HM. Stationery Oilice.

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From Cambridge Natural Hislary, vol. vii., “ Fishes, &c., ” by permission of Messrs. Macmillan & Co., Ltd.

FIG. 4.-Tfllapfla dolloi, a teleostean fish, to illustrate external features. (After Boulenger.)

A, Side view. g.r, Gill rakers. B, First bronchial arch. l.l, Lateral line organs. a.f, Anal fin. 11, Nasal opening. af, Caudal fin. p.f, Pelvic fin. d.f, Dorsal fin. pop, Preoperculum. g.f, Gill lamellae. pl.f, Pectoral lin. as in them we are compelled to recognize the homologies of the paired limbs of the higher vertebrates. We accordingly distinguish the two pairs of fins as pectoral or anterior and pelvic (==“ ventral ”) or posterior. There are two main types of paired fin-the archiptcryglal type, a paddle-like structure supported by a jointed axis which bears lateral rays and exists in an unmodined form in Neaceralorlus alone amongst living fishes, and the actinopterygial type, supported by nne raylike

structures as seen in the fins of any ordinary nsh. The relatively