. Text-book of embryology. Embryology. XVI ECHINODERMATA 473 coelom. The larval intestine is straight, and both it and the larval anus disappear shortly after the animal enters on its larval existence; that IS on the fifth day. The coelom is only separated from the gut after the stomodaeum has broken through. As for the second point (b), the larva has the form of a boot. The sole of the boot is the prae-oral lobe, v?hich is enormous, and the " upper " of the boot is the body. The back of the boot corresponds to the ventral surface of the larva, and here the larval mouth is situated;
. Text-book of embryology. Embryology. XVI ECHINODERMATA 473 coelom. The larval intestine is straight, and both it and the larval anus disappear shortly after the animal enters on its larval existence; that IS on the fifth day. The coelom is only separated from the gut after the stomodaeum has broken through. As for the second point (b), the larva has the form of a boot. The sole of the boot is the prae-oral lobe, v?hich is enormous, and the " upper " of the boot is the body. The back of the boot corresponds to the ventral surface of the larva, and here the larval mouth is situated; while the front of the boot is the dorsal surface. The prae-oral lobe is surrounded by a thickened ridge which bears specially long cilia, by the aid of which, and of the cilia of lesser length which cover the ectoderm everywhere, the larva glides about on the bottom. It uses the prae-oral lobe as a sucker, attaching the thickened rim to the substratum on which it is moving, and then retracting the centre. In the centre there appears, on the seventh Fig. 363.—Froutal longitudinal sections of two early embryos of Asterina gibbosa, in order to show the development of the coelom. (Original.) A, stage in which the coelom is a spherical vesicle. B, stage in wliich the coelom is growing back in tongues at the sides of the gut. al, rudiment of gut: hly^ blastopore; coe, rudiment of the coelom. a circular area of thickened glandular epithelium (Fig. 364). This is the fixing disc, and by means of the secretion produced by it the larva effects a permanent fixation to the bottom. Once this has been accomplished the rim is destroyed by the same process as that by which the brachiolar arms are removed in the Bipinnaria larva. The whole prae-oral lobe shrinks, until final atrophy takes place and the larva wrenches itself free and walks away as a little star-fish. The internal changes which take place during the larval life, and the metamorphoses, are known in detail. Let us go back t
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