40 W. WOODLAND. 



the plates of Holothuroidea is that described by me for the 

 Cucumariidse (Study IV), viz. the origin of the elongated 

 calcareous needle between two or four cells, its growth 

 to form a rod, the bifurcation of the extremities of this 

 rod, and so on. Up to the present I only know of one 

 exception to this rule, Semon (10) describing and figuring 

 most distinctly the triradiate mode of origin of certain 

 spicules in the holothurian Chiridota venusta. But this, 

 and possibly a few other exceptions, do not invalidate the 

 general rule, and, as before mentioned, this difference of 

 origin between most echinoderm spicules and the spicules of 

 holothurians can be correlated with a general difference 

 which exists between the modes of skeleton formation in the 

 two groups, i. e. this rule can be justified by a reason for its 

 existence. The quantity of lime respectively secreted by 

 most echinoderms and by holothurians differs greatly — in the 

 former group the stroma is packed with a calcareous stereom, 

 whereas in most individuals of the latter the skeleton is only 

 represented by isolated spicules — and correlated with this 

 diffei'ence is (o) the fact that in the former group every 

 scleroblast gives rise to a spicule, whereas in the latter at 

 least two scleroblasts have to co-operate for the same 

 purpose, and (&) the equally cogent fact that in most echino- 

 derms scleroblasts multiply very rapidly (shown by the 

 number of scleroblasts per spicule), whereas in holothurians 

 they multiply very slowly. In other words, the difference in 

 the origin of the spicule in the two groups is correlated with 

 the amount of the skeleton present — with the skeleton-pro- 

 ducing capacity. 



Previous work on the subject of the present paper, so far 

 as I have been able to discover, has been very small in 

 amount. In fact, the only paper that I know of describing 

 the origin of the spherical granule and the young triradiate in 

 a single scleroblast is that of Semon (10) on the holothurian 

 Chiridota just referred to. Semon also figures very dis- 

 tinctly the young triradiate with two scleroblasts (similar to 

 fig. 4). At the same time Semon represents the triangle- 



