Outlines of zoology . entation cavity; (b) from the outgrowingof one or more ccelom pouches (vaso-peritoneal vesicles)from the gastrula cavity or archenteron. From thesevesicles the body cavity and the rudiments of the water-vascular system arise. The larva is, first of all, a slightly modified, diffuselyciliated gastrula. In Holothuroids, Echinoids, Asteroids,and Ophiuroids, it becomes quaintly modified by theoutgrowth of external processes, and the formation of 276 PHYLUM ECHINODERMA. special ciliated bands. These are at first simply pre-oraland pre-anal rings, but they become drawn out alon
Outlines of zoology . entation cavity; (b) from the outgrowingof one or more ccelom pouches (vaso-peritoneal vesicles)from the gastrula cavity or archenteron. From thesevesicles the body cavity and the rudiments of the water-vascular system arise. The larva is, first of all, a slightly modified, diffuselyciliated gastrula. In Holothuroids, Echinoids, Asteroids,and Ophiuroids, it becomes quaintly modified by theoutgrowth of external processes, and the formation of 276 PHYLUM ECHINODERMA. special ciliated bands. These are at first simply pre-oraland pre-anal rings, but they become drawn out alongvariously disposed and shaped processes. The larva ofCrinoids (of Antedon) is not so divergent. In all casesthe bilateral symmetry is preserved. The larva does not grow directly into the adult. On thecontrary, the adult arises, for the most part, from newgrowth within the larva on one side. The arms or pro-cesses peculiar to the larva are absorbed or in part thrownoff. Only in a few forms which have brood-chambers or. Fig. 141.—Stages in development of Echinoderms.—After Selenka. I. Section of blastula of Synapta. digitata (Holothuroid), with a hint ofgaatrulation. 2. Section of gastrula of Toxopneustes brevispinosus (sea-urchin); ec.^ ectoderm; en., endoderm ; wz., segmentation cavity withmesenchyme cells in it. 3. Section of larva of Asterina gibbosa (star-fish); -S/., blastopore;^., archenteron ; z/./., vaso-peritoneal vesicle ;r. and /., right and left sides. are viviparous is the development direct, and without free-swimming larvffi. The celebrated comparative anatomist and physiologist, JohannesMiiller, was the first to show that the various types of EchinodermlarvEe might be derived from one fundamental form. This fundamental type is an elongated, oval, or pear-shaped larva,which is somewhat flattened on its ventral side. It has arisen from agastrula whose blastopore has become the anus, while the archenteronis bent towards the ventral surface, where it communicates
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
