ECHINOCYAMUS PUSILLUS. 45 



of calcification, the fifth in order, which gives rise, in a similar way to 

 that above mentioned, to a three-armed spicule, PI. VlJ^i fi,g. 92 — 96^ 

 the posterior arm of which remains short, while the two anterior ones 

 pass on each side of the oral area and enter two anterior dorsal arms, 

 which begin to protrude close by the anterior ventral arms, PL VI, 

 fig. 87 d. 



Thus, the mature Pluteus possesses eight arms and five centres 

 of calcification, PL F/, fig. 90. It measures, including the arms which 

 gradually attain a considerable length, about 0,72 mm. During the in- 

 crease of the young sea urchin, the Pluteus itself gradually changes 

 in its external shape. Its posterior portion, hitherto more or less 

 elongate, becomes more rounded or even shghtly broader than long, 

 and the angles between the posterior arms have by degrees travelled 

 backwards and are to be found nearer to the posterior end of the body. 

 Meanwhile, the ciliated band and the larval tegument in its vicinity in- 

 crease in such a degree as to cause the origin of a number of auricular 

 lobes, which shoot out from the body and grow slightly backwards, evi- 

 dently on purpose to facilitate the process of swimming and the loco- 

 motion of the young sea-urchin. In this manner the Pluteus finally 

 acquires three pairs of such ear-like lobes, PL VI, fig. 90 e, /, q. 



Inasmuch as the larval arms are capable of being stretched out 

 from the body, which process in its turn forces the auricular lobes to 

 extend in an almost horizontal direction, it is manifest that this arran- 

 gement assists movement in! the water in a high degree and acts in a 

 similar manner to the contractile swimming disc in the Cœlenterata. 



But also the foremost part of the larva has undergone some 

 changes, a small »preoral» lobe having been formed in front of the 

 ciliated band and between the anterior arms. 



As far as my experiences go, the development from the mature 

 Pluteus to the young sea-urchin takes a period of about six weeks, while 

 the changes from the egg to the mature Pluteus pass very rapidly or 

 in a fortnight. At least this is what has happened in my aquaria. 



A search into the rich literature which concerns the development 

 of the Echinoderms, shows that I. Müller alone has described larvœ of 

 Echinocyamus in different stages, dredged by means of a tow-net from 

 the surface of the sea. His drawings of larvœ obtained at Triest, Nice, 

 Elsinore and Heligoland evidently prove that he investigated young spe- 

 cimens of the Clypeastroid in question. In the first accounts in which 

 larvée of Echinocyamus are treated, I. Müller was in uncertainty as to 



