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Encyclopedia Britannica - Main :: PYR-RAY |
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RAY (Lat. raia) . The rays (Batoidei) together with the sharks (Selachoidei) form the suborder Plagiostomi of Elasmobranch fishes, and are divided into six families (see ICHTHYOLOGY). The first family, Pristidae, contains only the saw-fishes (Pristis), of which five species are known, from tropical and sub- tropical seas. They frequent es- pecially estuaries and river-mouths, and in some cases make their way over a hundred miles from the sea. Although saw-fishes possess all the essential characteristics of the rays proper, they retain the elongate form of the body
sharks, the tail being excessively muscular and the sole organ of locomotion. The " saw " (fig. 1) is a flat prolongation of the snout, with an endoskeleton which con- sists of three to five cartilaginous tubes; these are the rostral pro- cesses of the cranial cartilage and are found in all rays, though com- monly much shorter. The in- tegument of the saw is hard, covered with shagreen
series of strong teeth, sharp
front and flat behind, are em- bedded in it, in alveolar sockets, on each side. The saw is a for- midable weapon of offence, by means of which the fish tears pieces of flesh off the body
its victim, or rips open its abdomen to feed on the intestines. The teeth proper, with which the mouth is armed, are extremely small and obtuse, and unsuitable for wounding or seizing animals. Saw-fishes are abundant in the tropics; in their stomachs pieces of intestines and fragments of cuttle-fish have been found. They grow to a large size, specimens with saws 6 ft. long and 1 ft. broad at the base being common. The rays of the second family, Rhinobatidae, bear a strong resemblance to the saw-fishes, but lack the saw. Their teeth are consequently more developed, flat, obtuse, and adapted for crushing hard-shelled marine animals. There are about twenty known species, from tropical and subtropical seas.The third family, Torpedinidae, includes the electric rays. For the peculiar organ (fig. 2) by which the electricity is produced, see ICHTHYOLOGY. The fish uses this power voluntarily either to defend itself or to stun or kill the smaller animals on on the left side has been removed to show the electric organ. which it feeds. To receive the shock, the object must complete the galvanic circuit by communicating with the fish at two distinct points, either directly or through the medium of some conducting body. The electric currents created in these fishes exercise all the other known powers of electricity: they render the needle magnetic, decompose chemical compounds and emit the spark. The dorsal surface of the electric organ is positive, the ventral negative. Shocks from a large healthy fish will temporarily paralyse the arms of a strong man. The species of the genus Torpedo are distributed over the coasts of the Atlantic, Pacific and Indian Ocean, and at least one reaches the coasts of Great Britain (T. ~hebetans). On the west coast of North America T. californica occurs, while on the Atlantic coast there is found the black crampfish (T. occidentalis). This latter is said to reach a weight of ,200 lb, but such gigantic specimens are scarce, and prefer sandy ground at some distance from the shore, where they are not disturbed by the agitation of the surface-water. Seven genera with about fifteen species have been described, mostly from the warmer seas. All the rays of this family have, like electric fishes generally, a smooth and naked body. The fourth family, Raiidae, comprises the skates and rays proper, or Baia. More than thirty species are known, chiefly from the temperate seas of both hemispheres, but much more numerously from the northern than the southern. A few species descend to a depth of nearly 600 fathoms, without, however, essentially differing from their surface congeners. Rays, as is indicated by their shape, are bottom-fishes, living on flat sandy I perfectly flat molars, adapted for crushing hard substances. In ground, generally at no great distance from the coast or the surface. They lead a sedentary life, progressing, like the flat-fishes, by an undulatory motion of the greatly extended pectoral fins, the thin slender tail having lost the function
radiata
shagreen
The members of the fifth family, Trygonidae or sting-rays, are distinguished from the rays proper by having the vertical fins replaced by a strong spine attached to the upper side of the tail. Some fifty species are known, which inhabit tropical more than temperate seas, some species being found in great tropical rivers over x000 m. from the sea. The spine is barbed on the sides and is a most effective weapon of defence; by lashing the tail in every direction the sting-rays can inflict dangerous or at least extremely painful wounds. The danger arises from the lacerated nature of the wound rather than from any specially poisonous property of the mucus inoculated. Generally only one or two spines are developed. Sting-rays attain to about the same size as the skates and are eaten on the coasts of the Mediterranean and elsewhere. One species (Trygon pastinaca) is not rarely found in the North Atlantic and extends northwards to the coasts of Ireland, England and Norway. The rays of the sixth
some of the eagle-rays the molars are large and tessellated (fig. 4), similar to those of the sting-rays. In the enormous " sea-devils," sometimes classed as a separate family (Mobulidae), the anterior part of the pectoral fin is detached and forms a " cephalic " lobe or pair of lobes in front of the snout. The dentition consists of in others extremely small. Of the twenty-seven species which are known, from tropical and temperate seas, the majority attain a very large and some an enormous size: one mentioned by Risso, which was taken at Messina, weighed 1250 lb. A foetus taken from the uterus of the mother (all eagle-rays are viviparous), captured at Jamaica and preserved in the British Museum, is 5 ft. broad and weighed 20 lb. The mother measured 15 ft. in width and as many in length, and was between 3 and 4 ft. thick. At Jamaica, where these rays are well known under the name of " devil-fishes, " they are frequently attacked for sport's sake, but their capture is uncertain and sometimes attended with danger. The eagle-ray of the Mediterranean and Atlantic (Myliobatis aquila) is occasionally found off the British coasts. (A. C. G.; J. G. K.) End of Article: RAY (Lat. raia) If you wish, you can link directly to this article.
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