232 ECHINODERMA. 



formed in the young animal of a central plate surrounded 

 by five "ocular" and five " genital " plates. In the adult 

 the central plate is no longer distinct. The ocular or radial 

 plates bear eye specks ; the genital or basal plates bear the 

 apertures of the genital ducts, but one of the five is modified 

 as the madreporic plate. From pole to pole run ten 

 meridians of calcareous plates, which fit one another firmly ; 

 five of these (in a line with the ocular plates) are known as 

 ambulacral areas, for through their plates the locomotor 

 tube-feet are extruded ; the five others (in a line with the 

 genital plates) are called inter-ambulacral areas, and bear 

 spines, not tube-feet. Altogether, therefore, there are ten 

 meridians, and each meridian area has a double row of 

 plates. On the dry shell from which the spines have been 

 scraped, the ambulacral plates are seen to be perforated by 

 small pores, four pairs or so to each plate. Through each 

 pair of pores a tube-foot is connected with an internal 

 ampulla. In the star-fish the ambulacral areas are wholly 

 ventral, and the apical area seen on the dorsal surface of the 

 young forms is not demonstrable in the adult. 



On the shell there are obviously many spines, most 

 abundant on the inter-ambulacral areas. Their bases fit 

 over ball like knobs, and are moved upon these by muscles. 

 But besides these, there are two modified forms of spines, — 

 (a) the minute pedicellarias, with three snapping blades on 

 a soft stalk, and sometimes with apical glands; and (b) small 

 globular sphaeridia, which show some structural resemblances 

 to otocysts. It is said that, like true otocysts, they are con- 

 cerned with the perception of direction of motion. 



In front of the mouth project the tips of five teeth, which 

 move against one another, grasping and grinding small 

 particles. They are fixed in five large sockets, and along 

 with fifteen other pieces form "Aristotle's lantern,'"' a complex 

 masticating apparatus, of whose history we know little. It 

 surrounds the pharynx, and is swayed about and otherwise 

 moved by muscles, many of which are attached to five 

 beams which project inward from the margin of the shell 

 round about the mouth (Fig. 102). 



As in other Echinoderms, the skeleton of lime is meso- 

 dermic. The shell is covered externally by a delicate 

 ciliated ectoderm, beneath which, in a thin layer of con- 



