GROWTH OF NEW PLATES. 643 



parative size ; the .secondary plates are added either above, below, or later- 

 ally. In Strongylocentrotus they are added on the sides, new plates being 

 constantly intercalated between the upper secondary plate and the large 

 primary plate (PL VI f. t - 10), so as to form arcs varying in the number 

 of pores at different stages of growth. In Stomopneustes there are several 

 sets of secondary plates forming trigeminate arcs added on the side of the 

 large primary plate (PI. VI. f. ll, if). In Echinometra (PI. VI f. 12), the 

 number of secondary plates is smaller than in Strongylocentrotus; the mode 

 of growth is the same. In Parasalenia (PL VI f. 14), the arcs round the 

 base of the primary tubercles are formed by a combination of the structure 

 of Arbacia and Echinometra. In Heterocentrotus (PI. VI. f. 1,2), the arc, 

 made up of numerous pores, sometimes as many as thirteen, consists of 

 small secondary plates. In Sphaerechinus (PL VI f. 16-19) and Echino- 

 strephus, the formation of the arcs of pores resembles most closely Echino- 

 metra. 



