SEA-TJECHINS AND SEA-CtTOUMBEES. 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 

 water without relaxing its grasp. 



Looking at one of the first-named kind, the pediceUai-ia 

 triphylla, of this Echinus miliaris, 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 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. sphmra, 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, 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 



