SEA-URCHINS AND SEA-CUCUMBERS. 283 



skin of the Urchin ; that on the summit is placed a head 

 consisting of three 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, which lock into 

 each other; that the head-pieces (like the stem) consist of 

 calcareous centres, clothed with flesh; that, besides the 

 opening and shutting of the head, the stem can be swayed 

 from side to side ; and that all these movements are spon- 

 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 w T ater without relaxing its grasp. 



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

 triphylla, of this Echinus miliaria, we see that it consists 

 of three broad and thick somewhat triangular pieces, 

 jointed into a head, set on a thickish stem of transparent 

 gelatinous fibrous substance, through which a slender core 

 of calcareous matter runs that looks fibrous and blue. 

 The three movable pieces or blades are convex externally, 

 concave internally ; thin in substance, furnished along 

 their opposite or concave sides with two longitudinal 

 ridges or keels, each of which is cut into the most beau- 

 tifully 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. sphcei'a, 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 inclose a skeleton of calcareous 

 substance, glassy, colourless, and brittle, in which, ac- 

 cording to the plan I have already described, are ex- 

 cavated a multitude of oval cavities which form irregular 

 rows ; a central line runs down each piece, that is solid 



