METAZOA ECHINODERMA. 181 



shows the dark pavement spines finely, especially on 

 the dorsal side. Among these, on the ventral side, are 

 spines similar to those on which Colobocentrotus stands. 

 The longest club-shaped spines extend outward from the 

 sides of the corona, while young ones are seen just growing 

 from the upper surface. In the adult (Nos. 330, 331) the 

 pavement spines have longer cylindrical stems by which 

 they are attached, while the great club-shaped spines have 

 become formidable organs of defence. The size of these 

 organs is correlated with the increased thickness of the 

 corona. The oral area is large, having encroached upon 

 the corona. The ambulacra are broader on the lower 

 side than the interambulacra, while above the ambitus 

 they are narrow, and the pores are in narrow arcs of num- 

 erous pairs. 



Our common sea urchin, Strongylocentrotus drobachi- 

 ensis A. Ag. (PI. 332 ; No. 333, two spiny specimens and 

 a preparation of the shell), has the corona made of two 

 rowed ambulacra and interambulacra. Each ambulacrum 

 begins in the young with two plates, as shown in PI. 332, 

 fig. i, and each interambulacrum in one plate (fig. i, i'). 

 As the urchin grows older a portion of the ventral border 

 is resorbed, as shown in fig. 2, and the oral area is mem- 

 branous and spineless. No new rows of plates are added 

 in either the ambulacra or interambulacra, so that there 

 are only twenty rows of plates in all. These are shown 

 in the admirable preparation (No. 333), where each indi- 

 vidual plate has been separated and mounted. The 

 ambulacral plates show the arcs of pores which vary but 

 usually consist, of four or five pairs. These have arisen 

 from the unbroken vertical rows of pores of the young. 



The anal disc is seen to the right, made of tiny plates 

 which are placed together with considerable irregularity; 

 a little to one side of the center is the anus. The last 

 plate terminating each interambulacrum is a genital, the 

 largest of which is the madreporic body ; an ocular plate 

 is at the tip end of each ambulacral area. The five geni- 



