52 Hjalmae Théel, 



out three rods and takes the form of a three-armed spicule with equal 

 angles. Afterwards the ends of the arms are dichotomously branched, 

 and by a repetition of this process and the joining of the arms a plate 

 with large holes arises, which increases by new branches successively 

 protuding and uniting, fig. 97 — 98. In the meantime arms also begin to 

 grow in a vertical direction from the central part of the plate in order to 

 raise the tubercle of calcareous network. In the way that I have just 

 sketched the larger interradial plates originate and grow so as to as- 

 sume gradually the form familiar in the young sea-urchin. In these 

 plates, fig. 98, which measure about 0,07 mm. in diameter, the centrally 

 placed meshes first brought into existence are always considerably wider 

 than the peripheral ones originated later. 



As to the mode in which the spines develope, it will easily 

 be understood from the series of figures I have given, PL VII, fig. 

 99 a — k, for which reason I prefer to treat it very summarily. As 

 already mentioned, the spines, as well as the plates, begin as a minute 

 triangular deposit or rather tetrahedron placed on the exterior side of 

 the increasing plate and at some distance from it. The three angles 

 divide each in two, and thus the deposit changes into a minute star 

 having six obtuse arms and measuring about 0,oo4 mm. in diameter. 

 The arms increase in length until the star has reached a diameter of 

 0,012 mm., after which their ends become knobbed and gradually send 

 out two opposite processes which unite, and now we have a wheel-shaped 

 deposit. Simultaneously the centre protrudes a vertical spine. The 

 wheel gradually changes into a spine, which has a length of 0,i mm. in 

 the young sea-urchin in question. The basal portion of the spine, which 

 is enlarged so as to form a kind of head, becomes connected with the 

 plate by strong muscular fibres, which effect the movements in all the 

 directions of which the spines are capable. According to Ludwig') the 

 spines in Asterina gibbosa Forbes take their origin in the same way and 

 later Semon ^) described almost the same developmental process in Ar- 

 bacia pustulosa etc. 



As the young one grows larger, the spines increase and assume 

 gradually the final shape characteristic of those in the mature sea-urchin. 

 Thus, in the largest young ones I have succeeded in raising in my aquaria, 



1) Entwicklungsgeschichte der Asterina gibbosa. 1882. p. 177. pi. VIII. 



2) Beiträge zur Naturgeschichte der Synaptiden des Mittelmeers. 1887. p. 

 295. pl. 10. 



