Page 285 - Third Book of Reading Lessons

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284 THIRD BOOK OF
constructed as to present a considerable sur ce to the water, so that the blood may receive a su cient portion of the oxygen contajned in that element. As the water in contact with the gills becomes deterio­ rated, it" is necessary that a constant current be caused to ow over them. In most shes, t is is e ected by their taking the water in at the mouth, and expelling it om under the gill-covers. The blood, which is constantly sent to the branchi om the heart, is distributed by means of the arteries to eve part of the body, whence it returns to the heart by means of the veins.
3. The limbs are rmed into ns, the re-legs constituting what is termed the ec ral ns, and the posterior extremities, the ven al ns: besides these, ns, ordinary shes are rnished with one or' two dorsal ns, and a caiidal .
4. All these ns are not always present, nor when present, are they always in the same rela­ tive position: the absence of certain ns, and the peculiar position of these organs, a ord characters jn the classi cation of shes. The ns consist of a thin, elastic membrane, supported by rays. The rays are of two kinds: those which consist of a single bony piece, usually hard and pointed, are termed spinous rays, being long and thin, like a spine or thorn ; and
when the rays are rmed of numerous portions of bone, united by articulations, and equently divided longitudinally into several laments, they are called ex le rays. The principal organ of motion is the tail ; the dorsal and ventral ns apparently serve to balance the sh, and the pectoral, to arrest its pro- ,
gress when required.
5. The bones of shes are of a less dense and com- ;