COMMON STARFISH. 191 



specimen there are three rows of such spines, but in many instances there 

 are only two. At the oral end of each groove they form the mouth-papillae 

 above-mentioned, the spines of one side in one groove meeting the spines 

 of the adjoining side of the contiguous groove interradially. Externally 

 to this series of spines comes another series in triple row of stout spines, 

 fixed like all the other spines of the body. They are borne by the 

 median set of interambulacral ossicles (infra}. This series of spines, 

 and the series of moveable spines, both extend to the tip of the ray, 

 ceasing at a spot where a circlet of spines denotes the position of the 

 eye-speck and terminal feeler or tentacle in the living animal. A third 

 series of stout spines in a single row, borne by the inferior marginal 

 ossicles (infra}, borders the ventral aspect of the ray on either side. 



Turning to the dorsal or aboral surface the perisoma or integument 

 with its network of calcareous ossicles and membranous soft intervals 

 may be first noticed. The spine-bearing ossicles form more or less regular 

 lines parallel to the axes of the rays. One line in the middle must be 

 noted particularly. Attached to the soft intervals and at the bases or 

 tips of the spines may be seen scattered pedicellariae. Other pedicellariae 

 are grouped round and on the series of moveable spines of the ventral 

 surface. These two sets of pedicellariae differ remarkably from one 

 another. Both however are to be regarded as modified spines, not zooids, 

 like the polymorphic aviculariae and vibracula of Polyzoa. (See Flustra, 

 Preparation 48, post?} In one of the interradii is a circular calcareous plate, 

 the madreporic tubercle or madreporite, the surface of which is marked 

 by grooves radiating from the centre. By the removal of the perisoma 

 from the disc, it may be seen that a canal with calcareous walls the 

 stone-canal, curved like the letter S, leads from this plate to the ventral 

 surface, where it opens into the circum-oral water vascular vessel. The 

 two rays, one on either side the madreporic plate, constitute the bivium 

 as ordinarily defined ; the three remaining rays the trivium. The central 

 one of the three lies opposite the madreporic interradius and is often 

 spoken of as the anterior ray. 



The perisoma has been removed from the dorsal surface of three rays, 

 and the ambulacral ossicles can be seen from their dorsal aspect. Their 

 median ends form a prominent vertebral ridge with median furrow. 



Each first ambulacral ossicle is large, broad, and pointed medianly, and 

 projects over the peristome radially. It appears to be formed by the fusion 

 of two ossicles. The pore is large. At the outer end of this ossicle and 

 to the outer side of the pore is an enlarged first adambulacral ossicle, which 

 is, however, smaller than the ambulacral. This adambulacral with its 

 fellow adjoining the same interradius carries the oral papillae or spines 

 which project over the peristome interradially. In the majority of Starfish 

 the first adambulacral itself projects interradially over the peristome beyond 



