vii PHYLUM ECHINODERMATA 159 



along the middle of the ventral surface of one of the arms to 

 its extremity. Bordering each of these ambulacra I grooves 

 there are either two or three rows of movable calcareous 

 spines, the ambulacral spines. External to the ambulacral 

 spines are additional rows of stout spines, which are not 

 movable. 



On the convex dorsal surface there are a number of short 

 stout spines arranged in irregular rows parallel with the long 

 axes of the rays. These are supported on irregularly- 

 shaped ossicles buried in the integument. In the soft inter- 

 spaces between the ossicles are a number of minute pores, 

 the dermal pores, scarcely visible without the aid of a lens. 

 Through each of these pores projects a very soft filiform 

 process, one of the dermal branchice. or papula (Fig. 82, Resp. 

 cce.), which is capable of being entirely retracted. 



Very nearly, though not quite, in the centre of the 

 dorsal surface is an aperture, the anus (an.), wide enough to 

 admit of the passage of a moderately stout pin. On the 

 same surface, midway between the bases of the two rays, is 

 a flat, nearly circular plate, the surface of which is marked 

 by a number of radiating narrow, straight, or slightly wavy 

 grooves : this is the madreporite(mdpr.\ 



Attached to the spines of the ventral surface, in the 

 intervals between them, and in the intervals between 

 the spines on the dorsal surface, are a number of very 

 small, almost microscopic, bodies, which are termed the 

 pedicellaricz (Fig. 82, Fed.). Each of these is supported 

 on a longer or shorter flexible stalk, and consists of three 

 calcareous pieces, a basilar piece at the extremity of the 

 stalk, and two jaws, which are movably articulated with the 

 basilar piece, and are capable of being moved by a set of 

 muscular fibres, so as to open and close on one another 

 like the jaws of a bird. 



