264 PHYLUM ECHINODERMA. 



deposits. After the perils of youth are past, the largei 

 forms have few formidable enemies. 



' The hard and prickly body is more or less spherical. 

 _j_food canal begins in the middle of the lower surface ; 

 it ends at the opposite pole in the middle of an apical disc, 

 formed in the young animal of a central plate surrounded 

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

 the central plate is no longer distinct. Each of the 

 " oculars " has a hole for a sensitive tube-foot ; the genitals 

 bear the apertures of the genital ducts, and one also bears 

 the perforated madreporic plate. -\ From pole to pole run 

 ten meridians of calcareous platespwriich 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, v Altogether, therefore, there 

 , / are ten meridians, and each meridian area has a double 

 \J row of plates. *j/On the dry shell from which the spines 

 have been scraped, the ambulacral plates are seen to be 

 perforated by small pores, three pairs or so to each plate. 

 Through each pair of pores a tube-foot is connected with 

 an internal ampulla. In the starfish 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 kriobs, and are moved upon these by muscles. 

 But besides these, there are modified spines (a) several 

 kinds of pedicellariae, with three snapping blades on a soft 

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

 globular sphaeridia, which show some structural resem- 

 blances to otocysts. It is said that, like true otocysts, 

 they are concerned with the perception of direction of 

 motion. New spines and pedicellariae can be grown to 

 replace those that are shed in unwholesome conditions or 

 rubbed off by accident. This is the only marked regenera- 

 tion in sea-urchins. 



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 or pyramids, 



