ECHINOCYAMUS PÜSILLUS. 51 



larger plates occupying the interstices between the former. However, 

 it ought to be pointed out that in slightly older specimens the radial 

 plates seem to have united so as to form a single plate in each ambu- 

 lacrum. At least, I have not been able to discern more than one in the 

 three anterior ambulacra, viz. the frontal and the adjacent one on each 

 side of it, while in the two posterior there still remain some indications 

 of the previous state with double plates, PL IX, fig. 108. The changing 

 of the originally double plates into simple ones evidently takes place in 

 connection with the formation of the hollow chambers or outgrowths 

 of reticular tissue in which the spherids become concealed. According 

 to mj view, both of the two radial plates take part in these outgrowths 

 and become joined during this operation. As the spherids of the three 

 anterior ambulacra arise first, and subsequently those of the two poste- 

 rior, their calcified covering must originate in the same order, conse- 

 quently we have an explanation why the two plates of the posterior am- 

 bulacra have not as get had time to coalesce in the specimen above 

 mentioned. 



In the year 1855 Müller *) described and figured some young 

 Echinoderms which he supposed to be larvEe and young of Echinocya- 

 mus, but he did not pay attention either to the spherids or to the fact 

 that the radial plates were originally paired; on the whole, his figure of 

 the young sea-urchin does not seem to be very accurate. 



If we now return to the description of the young Echinocyamus, it 

 may be remarked that the five primary pedicels, PL VIII, fig. 106, which 

 we have already seen originate in the Pluteus, are distributed one in 

 each ambulacrum. Besides, other pedicels successively protrude in the 

 neighbourhood of the former though on the dorsal side of the boundary. 

 They are all devoid of supporting rods and terminal plates and their 

 tops present themselves as globular oval knobs slightly tapering to- 

 wards the end. 



Immediately after the first calcareous deposits of the future in- 

 terradial plates have become traceable, the spines which rest on the 

 tubercles of the plates and articulate with them, take their origin. Of 

 course the plates and the spines standing on their tubercles arise from 

 two different centres of calcification, PL VII, fig. 92 — 95. The first 

 indication of both consists in an extremely small triangular body or rather 

 tetrahedron originated by the agency of mesenchyme cells, PL VII, fig. 

 97 and fig. 99 a. The one which is destined to become a plate, sends 



1) Ueber die Gattungen der Seeigellarveu. 1855. p. 22 — 31. pi. VIII. 



