. Cassell's natural history. Animals; Animal behavior. NA TUliA L HISTOR two-fold function has been In a living Ophiurid a Fig. 12.—DIAGKAM ON A LONGITIDINAL SECTION TIIUOLGH THE DISC AND AN ARM-BASE OF AN OPHIUKID. (After H. Ludwi^.) an into the arm; ct, con- the aim-bases, that dovetail, as it were, into the disc, and ave visible on its under side, sepai-ate(l from one another by groups of regularlj'-arranged plates, which converge towards the central mouth. Each arm-base is separated from the plated interradial areas at its sides by slit-like openings, which are usually single, but occa
. Cassell's natural history. Animals; Animal behavior. NA TUliA L HISTOR two-fold function has been In a living Ophiurid a Fig. 12.—DIAGKAM ON A LONGITIDINAL SECTION TIIUOLGH THE DISC AND AN ARM-BASE OF AN OPHIUKID. (After H. Ludwi^.) an into the arm; ct, con- the aim-bases, that dovetail, as it were, into the disc, and ave visible on its under side, sepai-ate(l from one another by groups of regularlj'-arranged plates, which converge towards the central mouth. Each arm-base is separated from the plated interradial areas at its sides by slit-like openings, which are usually single, but occasionally double. These are known as the genital slits, and lead into thin walled pouches at the sides of the rays, to which assigned. double current of entrance and exit is visible around these genital slits, its cause appearing to lie in the alternate expansion and contraction of the disc ; and the pouches thus seem to serve as a kind of internal gills, or breathing apparatus. The water which enters them brings in oxygen, which it ex- changes for cai-bonic acid with the water in the body-cavity through the thin wall of the pouch, and then goes ^ out by the return current. The ovaries of the Ophiurids open into these pouches, and the ripe ova may either be carried out through the genital slits by the efferent currents, so as to undergo their larval metamorphoses independently of their parent, or they may remain within the pouches, and undergo a direct and more rapid development, as has been mentioned above. At the inner angle of each interradial area on the under surface of the disc is a plate known as the " mouth-shield " (Fig. 12, ms). Between each of these and the mouth is a complicated arrange- ment of plates, constituting what is called an oral angle (Fig. 12, oa, ta, (fee). At the apex of this are a number of short flat processes, the palece angulares (Fig. 12 pa), while its sides bear numerous smaller processes, the " ; These serve as
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Keywords: ., bookcentury1800, bookdecade1890, booksubjecta, booksubjectanimals
