SEA-URCHINS AND SEA-CUCUMBERS. 325 



mit is placed a liead consisting of tliree pieces, which 

 are capable of being widely opened and of being closed 

 together, at least at their tips ; that the edges of these 

 pieces, which come into mutual contact, are furnished 

 with teeth, wliich lock into each other; that the head- 

 pieces (like the stem) consist of calcareous centres, 

 clothed with flesh ; that, besides the opening and shut- 

 ting of the head, the stem can be swayed from side to 

 side ; and that all these movements are spontaneous, 

 and apparently voluntary. It appears that the head- 

 pieces close on any object presented to them, such as 

 the point of a needle, and hold with considerable force 

 and tenacity, so that the Pedicellaria may be drawn 

 out of the M'wter without relaxing its grasp. 



Looking at one of the first-named kind, the Pcd'icd- 

 lana trijphjlla^ of this JEdtinus miliaris, we see that it 

 consists of three broad and thick sub-triangular pieces, 

 jointed into a head, set on a thickish stem of transpar- 

 ent gelatinous fibrous substance, through which a slen- 

 der core of calcareous matter runs that looks fibrous 

 and blue. The three movable pieces or blades are con- 

 vex externally, concave internall}'; thin in substance, 

 furnished along their opposing or concave sides with 

 two lonjritudinal ridores or keels, each of which is cut 

 into the most beautifully fine teeth, so that the edge of 

 each ridge looks like a shark's tooth ; the edges of the 

 pieces are also similarly toothed : these shut precisely 

 into each other. 



In the larger K sphcera, the head-blades of this kind 

 have one stout central ridge, which is rounded and 

 not toothed. It forms the front of a great interior 

 cavity, which opens by two orifices on each side of the 

 3olumn. 



