XVIII 



ECHINUS PEDICELLARIAE 



507 



the head is due to the fact that there is on the outer surface of 

 each jaw a sac-like gland developed as a pouch of the ectoderm. 

 From it are given off two ducts which cross to the inner side of 

 the blades and, uniting into one, run in a groove to near the tip. 

 The gland secretes a poisonous fluid. The basal rod reaches up 





y mjj. 



^ jj.drif. 



Fig. 224. View of the apical region of Echinus esculentus, showing spines and pedi- 

 cellariae ; drawn from the living specimen, x 3. a, Anus ; g.p, genital pore ; 

 i, interradius ; mp, madreporite ; per, periproct ; p.gemm, gemmiform pedicellaria ; 

 pod, podia ; p.trid, tridactyle pedicellaria ; p.trif, trifoliate pedicellaria ; r, radius ; 

 t.t, pore for terminal tentacle of the radial water- vascular canal. 



to the jaws, so that this form of pedicellaria has a stiff stalk. 

 On the inner side of each blade, near the base, there is a slight 

 elevation (Fig. 225, B, s), consisting of cells bearing long cilia; 

 this is a sense-organ for perceiving mechanical stimuli. The 

 gemmiform pedicellariae are particularly abundant on the upper 

 surface of the animal. 



