SEA-URCHINS AND SEA-CUCUMBERS 321 



taneous, 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 

 water without relaxing its grasp. 



Looking at one of the first-named kind, the Pedicellaria 

 tripliylla, of this Echinus miliaris, we see that it consists 

 of three broad and thick sub- triangular pieces, jointed 

 into a head, set on a thickish stem of transparent gelat- 

 inous fibrous substance, through which a slender core oi 

 calcareous matter runs that looks fibrous and blue. The 

 three movable pieces or blades are convex externally, con- 

 cave internally; thin in substance, furnished along their 

 opposing or concave sides with two longitudinal ridges 

 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 E. 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 column. 



The movable pieces enclose a skeleton of calcareous 

 substance, glassy, colorless, and brittle, in which, accord- 

 ing to the plan I have already described, are excavated 

 a multitude of oval cavities which form irregular rows; a 

 central line runs down each piece that is solid and free 

 from cavities. This calcareous skeleton is incased in a 

 gelatinous flesh, similar to and continuous with that of the 

 stalk. 



This is the smallest kind, the head being about -g^th of 

 an inch in height. 



