Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/656

638 638 ICHTHYOLOGY [EXTERNAL rays remain jointed ; indeed, sometimes the foremost ray, with its preceding short supports, is likewise ossified, and is a hard spine, but the articulations can nearly always be distinctly traced. Sometimes the dorsal fin of Malaco- pterygian fishes is very long, extending from the head to the end of the tail ; sometimes it is reduced to a few rays only; and in a few cases it is entirely absent. In addition to the rayed dorsal fin, many Malacopterygian fishes (as the Salmonoids, many Siluroids, Scopeloids, &c.) have another of greater or lesser extent, without any rays ; and as fat is always deposited within this fold, it is called a fatty fin (pinna adiposa) (fig-*. 8 and 9). FIG. 9. Salmo salar (Salmon), with abdominal ventral fins. The anal fin is built on the same plan as the dorsal; it may be single, or there may be more than one; it may also be long or short, or entirely absent : in Acanthopterygians its foremost rays are frequently simple and spinous. FIG. U.Mitllus barbatus (Red Mullet), with thoracic ventral fins. The horizontal or paired fins consist of two pairs, the pectorals and ventrals. The pectoral fins (with their osseous supports) are the homologues of the anterior limbs of the higher Vertebrata. They are always inserted immediately behind the gill opening, either symmetrical, with a rounded posterior FIG. 11. Burbot (Lota vu 7 garis), with jugular ventral fins. margin, or asymmetrical, with the upper rays longest and strongest ; in Malacopterygians with a dorsal spine the upper pectoral ray is frequently developed into a similar defensive weapon. The ventral fins are the homologues of the hind-limbs, and are inserted on the abdominal surface, either behind the pectorals (Pisces or Pinnae abdominales) (fig. 9), or below them (Pisces or Pinnae thoracicce} (fig. 1U), or in advance of them (Pisces or Pinna; jugulares) (fig. 11). They are generally narrow, and composed of a small number of rays, the outer of which is frequently osseous. For the definition of the smaller systematic groups, and the determination of species, the numbers of the spines and rays are generally of the greatest importance. This holds good especially for the ventral rays, by the number of which the Acanthopterygian affinities of a fish can nearly always be determined. The numbers of the dorsal and anal rays generally correspond to the number of vertebrae in a certain portion of the spinous column, and are therefore constant specific, generic, or even family characters ; but when their number is very great, a proportionally wide margin must be allowed for variation, and the taxonomic value of this character becomes uncertain. The numbers of the pectoral and caudal rays are rarely of any account. The fins are organs of motion, but it is chiefly the tail and the caudal fin by which the fish impels itself forward. To execute energetic locomotion, the tail and caudal fin are strongly bent with rapidity, alternately towards the right and left ; whilst a gentle motion forwards is effected by a simple undulating action of the caudal fin, the lobes of which act like the blades of a screw. Retrograde motions can be made by fishes in an imperfect manner only, by for ward strokes of the pectoral fins. When the fish wants to turn towards the left, he gives a stroke of the tail towards the right, the right pectoral acting simultaneously, whilst the left remains pressed close to the body. Thus the pectoral fins assist in the progressive motions of the fish, but rather by directing its course than by acting as power ful propellers. The chief function of the paired fins is to maintain the balance of the fish in the water, which is always most unsteady where there is no weight to sink it: when the pectoral of one side, or the pectoral and ventral of the same side, are removed, the fish loses its balance and falls on the side opposite; when both pectorals are removed, the fish s head sinks ; on removal of the dorsal and anal fins the motion of the fish assumes a zigzag course ; de prived of all fins, it floats like a dead fish, with the belly upwards, the back being the heavier part of the body. In numerous groups of fishes which live in mud, or are able to pass a longer or shorter time in soil periodically dried and hardened during the hot season, forms occur en tirely devoid of, or with only rudimentary, ventral fins (Cyprinodon, Ophiocephalidce, Galaxiidce, Siluridce). The chief function of these fins being to balance the body of the fish whilst swimming, it is evident that in fishes moving during a great part of their life over swampy ground, or through more or less consistent mud, this function of the ventral fins ceases, and nature can readily dispense with these organs altogether. In certain fishes the shape and function of the fins are considerably modified: thus, in the rays, locomotion is almost entirely effected and regulated by the broad and expanded pectoral fins acting with an undulatory motion of their mar gins, similar to the undulations of the long vertical fins of the flat-fishes; in many blennies the ventral fins are adapted for walking on the sea-bottom ; in some Gobioids (Periophthal- FIG. 12. PeHophtUa inus kodreutcri. mus), Trigloids, Scorpajnioids, and Pediculati the pectoral fins are perfect organs of walking ; in the gobies, Cyclopferi, and Discoboli the ventral fins are transformed into an ad hesive disk (see fig. 13) ; and finally in the flying-fish the pectorals act as a parachute. In the eels and other snake- like fishes, the swimming as well as the gliding motions are effected by several curvatures of the body alternately to-