ECHINODERM LARVAE. 487 



progressive stages of the development of the "Echinus 

 rudiment " as it is called by MacBride (10), within the 

 echinopluteus. In figs. 51, 52, and 53, this rudiment is 

 seen as a dark, discoid mass closely applied to the left 

 side of the large, sub-globular stomach. At the stage 

 shown in fig. 53, the arms of the echinopluteus are already 

 shortening; and on the right side of the body a single 

 sessile pedicellaria (pd.) has appeared. In the next stage 

 figured (fig. 55), the number of pedicellariae has increased 

 to three; and in addition to numerous spicular plates 

 (sh.p.) — rudiments of the plates of the test of the adult — 

 developed on the oral face of the echinus rudiment, a 

 number of minute spines (sp.) project from its surface. 

 In fig. 54, a stage is shown in which the echinus rudiment 

 has assumed the definite, subpentangular form of the 

 adult. The peristomial aperture of the future test is 

 clearly visible; and the five primary tube-feet (u.t.) are 

 present, each with a rudimentary rosette in its terminal 

 sucker. No more interesting stage was observed than 

 that shown in fig. 56. In this a pair of tube-feet (t.f.) 

 is present in each radius in addition to the unpaired one 

 (u.t.) just indicated in fig. 54. In this respect the meta- 

 morphosed larva of E. miliaris differs from that of 

 E. esculentus, in which only the unpaired one is present. 

 At this stage two distinct kinds of spines are easily recog- 

 nised. Some are similar to those of the adult, inasmuch 

 as they taper gradually from base to bluntly pointed end. 

 These are confined to the oral face of the young Echinus. 

 Others are of almost uniform thickness throughout their 

 length, and the free end is produced into three or four 

 minute points. These are borne upon the convex aboral 

 face. The adult mouth (ad.m.) is just discernible at this 

 Btage, but does not appear to be perforate. l ; i^\ 57, 

 PI. IX, shows an aboral view of a post-larval bui sfill 



