264 PHYLUM ECHINODERMA. 
deposits. After the perils of youth are past, the larger 
forms have few formidable enemies. 
The hard and prickly body is more or less spherical. 
The 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. .In the adult 
the central plate is no longer distinct. Each has a hole 
for the protrusion of 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 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, 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 knobs, and are moved upon these by muscles. 
But besides these, there are modified spines—(a) several 
kinds of pedicellarize, with three snapping blades on a soft 
stalk, and sometimes with apical glands; and (4) small 
globular sphzeridia, 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 pedicellarie 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, 
