402 



COMPARATIVE ANATOMY 





way that its prongs point down towards the peristome over the edge of the base of 



of the masticatory apparatus (Fig. 848,.-We 

 here'refer back to what has been said of the perignathous apophysial ring, for the 

 ^^ and the apophysial ring are, physiologieally, closely connected. 

 Th most Important muscles and bands of the masticatory apparatus connect its 

 component pieces with the apophysial ring, and the latter must be regarded merely 

 as a folding inwards of the edge of the peristome, which has come into existence 



Fin. 348. Masticatory apparatus of an Echinoid (Toxopneustes) in its natural position at the 

 centre of the oral side of the shell, which has been broken off, original. 1, Root of the tooth ; 2, 

 intestine ; 3, accessory intestine (?) ; 4, axial sinus with stone canal ; 5, forked radii ; 6, arcus of a 

 single pyramid ; 7, muscles of the forked radii ; 8, perignathous apophysis (auricula) ; 9, ligaments of 

 the forked radii ; 10, adductor muscles of the teeth ; 11, opening muscles of the teeth ; 12, radial 

 canal of the water vascular system ; 13, ampullae ; im, interambulacrum ; am, ambulacrum. The 

 delicate transparent lantern membrane which covers the whole of the masticatory apparatus is 

 not represented. 



for the insertion of the masticatory muscles. The two apparatus are either absent or 

 present simultaneously. 



Round the masticatory apparatus, ten thin ligaments (9) connect the forked radii 

 with the interradial apophyses of the perignathous girdle. The two bands which 

 belong to each fork are attached to the prongs, and continue their lines down- 

 wards to the apophysial girdle ; they are inserted into the two neighbouring inter- 

 radial apophyses near the interradial sutures. 



The two bands proceeding from each radial fork thus diverge downwards, and 

 the two proceeding from each interradial apophysis of the perignathous ring diverge 

 upwards. 



These bands appear merely to serve for the attachment of the masticatory appa- 



