XIV] ECHINOIDEA 349 



kind, termed tridactyle, has long jaws and a flexible stalk. It 

 was supposed that these helped the animal to climb by seizing hold 

 of waving fronds of seaweed till the tube-feet could get a hold, but 

 this is proved not to be the case. It has been shown that a gentle 

 movement in the water excites the tridactyle pedicellariae while a 

 stronger movement calls the gemmiform into activity. The most 

 probable use of the tridactyle pedicellariae is to destroy minute 

 organisms (often the larvae of sessile animals) which would either 

 attach themselves to the skin of the Urchin or burrow into it. 

 Besides these there are two other kinds of pedicellariae. In one of 

 these the so-called "snake-headed" or ophicephalous pedicel- 

 lariae, the jaws are broad and fringed at the edges with powerful 

 teeth. They articulate with one another at their bases by means 

 of stout semicircular hoops which fit inside one another. This 

 arrangement gives to the jaws a firm grip and this kind of pedi- 

 cellaria which is scattered all over the surface of the Sea-urchin 

 and is especially abundant on the peristome is the only kind 

 which has been observed to seize and hold small animals such as 

 shrimps which accidentally brash against the Sea-urchin. These 

 are held till the tube-feet can be brought into play, when they are 

 carried round to the mouth and devoured. The last kind of pedi- 

 cellariae which the Urchin possesses are the most minute of all. 

 They are termed trifoliate pedicellariae; each possesses three 

 blades much broader at the free end than at the base of the 

 shape in fact which is termed by botanists obovate. Two of these 

 blades can seize an object whilst the third blade strikes it. The 

 object of the trifoliate pedicellariae is to free the delicate skin 

 of the Urchin from the sediment which falls on it from turbid 

 water. The lumps of this sediment are broken into the finest dust 

 by these pedicellariae, and this dust is then swept off by the cilia 

 covering the skin. 



The Urchin is provided with five white chisel-like teeth, each of 

 which is firmly gripped by a pair of grooved pieces called alveoli, 

 meeting in a point below. Each pair of alveoli meet in a point 

 where they clasp the tooth. Above they are united by two pieces 

 called epiphyses (13, Fig. 162) which meet in an arch. A pair of 

 alveoli with their epiphyses are oft,en spoken of as a jaw, and 

 adjacent jaws are joined by stout, inwardly projecting rods called 

 rotulae. The whole apparatus of five jaws has received the name 

 of "Aristotle's Lantern." This can be pushed out or pulled in 

 by muscles attached to arches called auriculae, rising from the 



