io8 THE BIOLOGY OF THE SEA-SHORE 



of the starfish touch the head of a pedicellaria than it is 

 immediately bitten, the pedicellaria remaining in the 

 wound. The effect of such bites is to cause the starfish 

 to withdraw. The first *' round " invariably ends in favour 

 of the sea-urchin. If the starfish renews the fight the 

 sea-urchin will eventually succumb owing to the loss of 

 its weapons. 



A point of great interest is the fact, emphasised by 

 Prouho, that in such a conflict as the above, the movement 

 of the spines away from the threatened area, is such as to 

 leave the pedicellariae a clear field of action, and is the exact 

 opposite of the convergent movement of the spines pro- 

 duced when the surface of the test is stimulated by an 

 instrument such as a needle. 



Weapons acting purely mechanically, — ^We meet with no 

 examples of these below the Echinoderms. In this group 

 (Echinoids) a last form of pedicellariae still remains uncon- 

 sidered, namely, the ophicephalous pedicellariae. These 

 occur over the whole surface of the test, but are most 

 numerous in the region of the mouth, and are specialised 

 for taking a firm grip. Their function is to assist in holding 

 small animals such as Crustacea till the tube feet can reach 

 them and convey them to the mouth. Strictly speaking, 

 therefore, the ophicephalous pedicellariae are concerned with 

 nutrition. Just as it is difficult to draw a sharp line between 

 weapons of attack and defence, so it is impossible to dis- 

 sociate weapons in the strict sense from jaws, teeth, and 

 similar organs which, though chiefly concerned with masti- 

 cation, may be turned to other uses if occasion requires it. 

 For instance, the pharynx of Nereid worms bears on its 

 muscular inner walls, in addition to numerous small spines, 

 a pair of powerful chitinous jaws. When the pharynx is 

 protruded the jaws become completely terminal, and are 

 used for seizing prey. If one of these forms is picked up 

 in the hand the jaws are at once brought into action, and 

 form powerful weapons of defence. The jaws thus serve 

 the triple purpose of mastication (when the pharynx is 

 retracted), attack, and defence. 



