. Text book of zoology. Zoology. 126 Echinoderma. in tlie epidermis, wliilst in other forms they have sunk farther in. As to sense organs, the small eyes present at the tips of the arms of Starfish must be mentioned. Small eyes are also present in certain Holothurians {Synapta), at the base of the tentacles; and lastly, optic organs have been described in some of the Sea-urchins where they are distributed in larger numbers over the whole surface of the body. Yesicular auditory organs are known only in some of the Holothurians. Reproduction, with a few exceptions, is sexual, and the Echinoderms
. Text book of zoology. Zoology. 126 Echinoderma. in tlie epidermis, wliilst in other forms they have sunk farther in. As to sense organs, the small eyes present at the tips of the arms of Starfish must be mentioned. Small eyes are also present in certain Holothurians {Synapta), at the base of the tentacles; and lastly, optic organs have been described in some of the Sea-urchins where they are distributed in larger numbers over the whole surface of the body. Yesicular auditory organs are known only in some of the Holothurians. Reproduction, with a few exceptions, is sexual, and the Echinoderms are almost all of separate sexes. The male and female organs are usually very similar in form, but they may generally be distinguished without microscopic examination, since the ovary is yellow or red, the testis white. As a rule they are radially arranged, in each inter-radius an ovary or testis, or a small group of these; some- times they are absent from one or more inter-radii, as in Irregular Sea-urchins; or they may be present in one only, as in the Holothurians. They may be saccular or branched, and each opens by a pore upon its inter-radius; in some forms near to the aboral pole, in others at some distance from this; or again, quite close to the mouth. Fertilisation generally occiu-s after the deposition of the ova, whicli are usually small. Some few Echinoderms ai-e, however, viviparous, and in these, of coui'se, fertilisation takes place within the body of the parent. In some forms the ova ai-e can-ied about by the parent, either protected by the spines or in special pits in the siu-face of the body; some Starfish foita a kind of brood-poTich by bending the arms down over the eggs. The development of the Echinoderms is of special interest, for a complicated metamorphosis often occurs: the larval. rig. 80. Diagrammatic figures of the principal forms of young Echinoderm larvte. A, B, C seen from below; A' i& A from the left side, a anus, / ciliated ridge, m month. T
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
