Outlines of zoology . h three snapping blades on a softstalk, and sometimes with apical glands; and (b) smallglobular sphseridia, which show some structural resem-blances to otocysts. It is said that, like true otocysts,they are concerned with the perception of direction ofmotion. New spines and pedicellarise can be grown toreplace those that are shed in unwholesome conditions orrubbed off by accident. This is the only marked regenera-tion in sea-urchins. In front of the mouth project the tips of five teeth, whichmove against one another, grasping and grinding smallparticles. They are fixed in
Outlines of zoology . h three snapping blades on a softstalk, and sometimes with apical glands; and (b) smallglobular sphseridia, which show some structural resem-blances to otocysts. It is said that, like true otocysts,they are concerned with the perception of direction ofmotion. New spines and pedicellarise can be grown toreplace those that are shed in unwholesome conditions orrubbed off by accident. This is the only marked regenera-tion in sea-urchins. In front of the mouth project the tips of five teeth, whichmove against one another, grasping and grinding smallparticles. They are fixed in five large sockets or pyramids, SEA-URCHIN. 26:; and along with five stout braces- (rotulae) and five curvedcompasses (radii) form Aristotles lantern, a complexmasticating apparatus, of whose history we know little. Itsurrounds the pharynx, and is swayed about and otherwisemoved by muscles, many of which are attached to fivebeams which project inward from the margin of the shelland form a girdleround about the Fig. 137.—Dissection of sea-urchin. M. at the lower pole is the mouth; M. at the upper pole is the madreporicplate; 7. 7., one of the large tentacular tube-feet around the mouth;^ a skin-gill; 57., a standard or perignath; AL.^ an alveolus;7?. a radial vessel, with ampullx {A,), intestine (iftt.) fixed by-mesenteries; P., a pedicellaria; C, a gonad: SP,, spines; T,F.,tube-feet. As in other Echinoderms, the skeleton of lime is meso-dermic. The shell is covered externally by a delicateciliated ectoderm, beneath which, in a thin layer of con-nective tissue, there is a network of nerve fibres, and someganglion cells. Internally, there is anotherthin layer ofconnective tissue, and a ciliated epithelium lining thebody cavity. The skeleton grows by the formation of 266 PHYLUM ECHINODERMA. new plates around the apical disc, and also by the indi-vidual increase of each. In a few forms the shell retainssome plasticity. The nervous system consists of a ring around
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
