5o8 ECHINODERMATA ECHINOIDEA chap. 



(3) "Trifoliate" (Fig. 225, E; Fig. 226, D): these are very 

 small pedicellariae, in which the jaws are shaped like leaves with 

 the broad end projecting outwards. They are scattered over the 

 whole surface of the body. 



(4) "Ophicephalous" (Fig. 225, D ; Fig. 226, C): pedicellariae 

 in which the jaws have broad rounded distal ends fringed with 

 teeth ; these ends bear a resemblance to a snake's head, whence 

 the name. The bases are also broad and thin, with a strong 

 median rib and a peculiar semicircular hoop beneath the spot 

 where they articulate with one another. The three hoops of the 

 three jaws work inside each other in such a way as to cause the 

 jaws to have a strong grip and to be very difficult to dislocate 

 from their mutual articulation. 



The ophicephalous pedicellariae are in Echinus the most 

 abundant of all; and tliey alone extend on to the peristome, 

 where a special small variety of them is found. 



A thorough investigation of the functions and reactions of 

 the pedicellariae has quite recently been made by von Uexkiill.^ 

 He showed, first of all, that there is a nervous centre in the stalk 

 of each pedicellaria (see below), which causes the organ to 

 incline towards a weak stimulus, but to bend away from a stronger 

 stimulus. In the head there is an independent nervous centre, 

 which regulates the opening and closing of the valves, and causes 

 these to open on slight stimulus and close when a stronger one 

 is applied. The amount of stimulus necessary to cause the 

 pedicellariae to retreat varies with the kind of pedicellariae, 

 being least with the tridactyle and most with the gemmiform, 

 so that when a chemical stimulus, such as a drop of dilute ammonia, 

 is applied to the skin, the tridactyle pedicellariae may be seen to 

 flee from and the gemmiform to approach the point of stimula- 

 tion. In a living Sea-urchin, if the attempt is made to seize 

 the tridactyle pedicellariae they will evade the forceps, but the 

 ophiocephalous are easy to catch. 



The tridactyle pedicellariae open with the very slightest 

 mechanical stimulus and close with rather greater mechanical 

 stimuli or with exceedingly slight chemical ones. Uexkiill calls 

 them " Snap-pedicellariae," and their function is to seize and 

 destroy the minute swimming larvae of various sessile parasitic 



' Uexkilll, " Die Physiologie der Pedioellarien," Zeitschr. fur Biol, xxxvii. 1899, 

 p. 334. 



