BTHAN(4 TILL K. SV. VBT.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 13 



the corona will form and in its growth raise np above the 

 ventral region the whole of the calycinal system along with 

 the provisional radial suckers. What precedes has shown that 

 this is the course of the development in the Cidarid; from 

 what follows it will be seen that an analogous process main- 

 tains in the Ectobranchiates. 



The fig. 25, 26 on Fl. IV represent the ventral part of 

 a denuded speeimen referable, it will seem, to the Strongylo- 

 centrus dröbakensis O. F. M. It measures 1,2 mm. in dia- 

 meter and has been cut through horizontally and preserved 

 in Canada balsam. En the centre. fifl- 25., is seen the oeso- 

 phagns with its hve lobes. The corona presents in each 

 ambulacrum four pairs and a half of plates. Of these the 

 first-formed, primordial pairs, already separated from those 

 of the second eircle. are placed each in front of its hxed am- 

 Imlacrum. and still very near it: two expanded oval plates, 

 angularly converging and contignons near their aboral ends, 

 diverging adorally. They conspicuously exhibit the hetero- 

 tropic disposition, the I a, II a, III h, IV «, V h still being 

 without pedicellar pores. while such are present in the I &, II 

 6, III ft, IV h, Va.' The second eircle, the exactly circular 

 peristome, now consists of the ten paired second ambnlacrals 

 and the hve well-developed primary interradials, solitary as 

 in the Cidarid, which have entered into the eircle and are 

 foUowed each by a pair of hexagonal. alternating. strong, 

 convex plates. 



The stoma is covered, like that of the vonns; Goniocida- 

 ris, by the lar val envelope, which is thin and so transparent 

 as not to be made distingnishable in the drawing. The dental 

 apparatus is readily seen through it. Each p^-ramid has its 

 halves no longer separated. l)ut united together. The tooth is 

 forming, but the keel alone is seen projecting beyond the la- 

 teral parts, which are not yet so far developed.- The sides 

 of the pyramids are not flat, as they will })e in the adult, but 

 (joncave, and hence the inter-pyramidal muscles are very long. 

 The two pairs of external motor muscles are present. The 

 two retractors of each pyramid, m. re., come into view each 

 from under the peristomal margin of a difl^^erent ambulacrum, 

 at its nearest point, but are wide apart, being separated by 



' Compare the Etudes s. i. Echinoidées, p. "27. Pl. XVII, lig. 148. 



^ Compare Joh. Mueller, Metamorphose, I, 1848, p. 24, Pl. VII, fig. 11. 



