566 THE INVERTEBRATA 



abuts upon the periproct and bears the opening of a gonoduct. One 

 of the genital plates bears also the madreporite. All the plates are 

 studded with bosses of various sizes, to which articulate the concave 

 bases of the large and small spines and the pedicellariae. The spines, 

 unlike those of starfishes and brittle stars, which are moved with the 

 ossicles under them, have muscles of their own. These are in two sets, 

 an outer one which causes movements, and an inner one which holds 

 the spine firmly in position. On level ground the spines take part at 

 times in locomotion, the animal using them like stilts. The pedi- 

 cellariae (Fig. 424), which have three jaws, are of several kinds. 

 Gemmiform pedicellariae have stiff stalks and globular heads with a 

 poison gland in each jaw. The tridactyle kind have a flexible stalk and 



Fig. 424. Pedicellariae of Echinus miliaris. Enlarged, but not accurately 

 to scale. A, Trifoliate. B, Ophiocephalous. C, Tridactyle. D, Gemmiform. 



long jaws. The ophiocephalous kind are smaller and have a flexible 

 stalk and broad, toothed jaws. The trifoliate pedicellariae are the 

 smallest and have very flexible stalks and broad, blunt jaws. It is said 

 that the gemmiform kind are weapons of defence against large foes, 

 the tridactyle against small, the ophiocephalous seize small animals 

 for food, and the trifoliate destroy debris. The peristomial edge of 

 the corona is indented in each interradius by two notches, where stand 

 the gills — delicate, branched outgrowths of the body wall, each con- 

 taining a cavity which is continuous with the lantern coelom (see 

 below). Ten little plates on the peristome around the mouth carry 

 openings for the ten short, stout, sensory buccal tube feet, the proxi- 

 mal pair of podia in each radius. 



The mouth, which is surrounded by five strong, slightly projecting, 

 chisel-shaped, interradial teeth, leads into a relatively narrow oeso- 

 phagus, whose lower part is enclosed in a framework, known as 



