ON THE YOUNG STAGES OF ECHINI. 737 



smallest specimens a few large spines (PL VIII. f. ie), resembling the spines 

 of young Dorocidaris hystrix, equalling in length the diameter of the test. 

 As the specimens increase in size, the spines lose their spindle-shaped form 

 and their serrate edge ; they become more pointed and elongate (PI. VIII f. 

 5, 12, is), diminishing rapidly in proportion to the size of the test, and soon take 

 the proportions they have in the adult. The actinal opening is very large at 

 first, the test in young specimens being a narrow ring when seen from the 

 actinal side (PI. VIII. f. ;). The primary tubercles are few in number, with 

 remarkably prominent ridges radiating from them (PL VIII. f. 14), leaving 

 deep pits between the ridges. With increasing size these ridges become 

 miliaries and secondary tubercles, the pits, however, remaining round the 

 boss of the primary tubercles in both the areas (PL VIII. f. 4, 11); so that 

 the test passes through stages in which it resembles at first young Echinus, 

 then an Echinus with deep grooves radiating from the tubercles, and finally 

 with deep pits round their base. The original subanal plate (PL VIII f. 10) 

 retains always its preponderance (PL VIII f. .;), and the embryonic charac- 

 ter of the anal system is a marked feature of this interesting sea-urchin. The 

 actinal opening rapidly becomes smaller (PL VIII f. 2, 9, It), and resem- 

 bles that of Echinus. In fact, Temnechinus might be called an Echinus 

 among Temnopleuridae, while Temnopleurus is the Toxopneustes of the 

 family. 



The changes taking place in the arrangement of the pores in Hipponoe 

 and Toxopneustes are similar to those observed in Echinus: at first a simple 

 vertical row, then arcs laterally curved (PL VIII J. ;u). then three pairs of 

 pores for each ambulacral plate, in oblique open curves, and finally almost 

 horizontal curves, the pores appearing to be placed in independent vertical 

 rows. 



Among the Clypeastroids we find in the young during their growth great 

 changes of form and structure taking place. In young Echinarachnius (PL 

 XII f. 1-4) the outline is elliptical, the test is arched and high, the anus is 

 placed in a slight depression of the test, and, seen in profile (PL XII f. 3), 

 we are reminded of the general aspect of Pygorhynchus. There are but 

 two principal rows of large tubercles in each area, extending from apex to 

 mouth, so that, seen from above (PL XII. f. 1), the young Echinarachnius 

 has much the facies of an Echinometra (PL XII f. 2). The mouth is large, 

 pentagonal, its radius being half the radius of the test. The ambulacral 

 rosette is reduced to two pairs of pores, — simple perforations of the test. 



