XVIII 



ECHINUS PEDICELLARIAE 



509 



animals, which would otherwise settle on the delicate exposed 

 ectoderm of the Sea-urchin. 



The gemmiform pedicellariae are brought into action when a 

 more serious danger threatens the Sea-urchin, such as an attack 

 of a Starfish. The corrosive chemical influence, which it can 

 be proved exudes not only from the stomach but even from the 

 tube-feet of the Starfish, causes the gemmiform pedicellariae to 

 approach and open widely. When the foe approaches so closely 

 as to touch the sense-organs (Fig. 225, B, 5) situated on the inner 



/t&^K 



FlG. 225. The pedicellariae of Echinus acutus, drawn from a living specimen. A, 

 gemmiform pedicellaria, closed. B, gemmiform pedicellaria, open ; g, poison gland ; 

 s, sense-organ, x 3. C, tridactyle pedicellaria, x 6. D, ophicephalous pedicellaria, 

 x 9. E, trifoliate pe*dicellaria, x 12 ; a (in all figures), axial rod of the stalk. 

 (After Uexkiill.) 



side of the valves of these pedicellariae, the blades close violently, 

 wounding the aggressor and causing its juice to exude, thus 

 producing a renewed and severe chemical stimulation which 

 irritates the poison glands and causes the poison to exude. The 

 virulence of the poison maybe gauged from the fact that the 

 bite of a single gemmiform pedicellaria caused a frog's heart to 

 stop beating. 



Prouho 1 has described a combat between a Sea-urchin and 

 a Starfish. When the latter approached, the spines of the Sea- 



1 ,; Du role des pedicellaires gemmiformes des Oursins," Compt. Rend. Acad, de 

 Paris, cxi. 1890, pp. 62-64. 



