vii PHYLUM ECHINODERMATA 159 



to its extremity. Bordering each of the ambulacral 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 interspaces 

 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 branchia or papula (Fig. 88, 

 Resp. eai), 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 sur- 

 face, 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 madrepoi-ite. 



Attached to the spines of the ventral surface, in the inter- 

 vals 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 pcdicellaricE (Fig. 

 88, 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 certain set of muscular 

 fibres, so as to open and close on one another like the jaws 

 of a bird. 



In a well-preserved specimen there will be seen in each 



