STUDIES IN SPICULE FORMATION. 555 
spicule there is thus observable a rough relation between size 
of spicule and the number of attached scleroblasts, and such 
is the case with the similar type of spicule of Cucumaria. In 
the large spicules of Cucumaria, it will be remembered, I was 
unable to trace any such relation, but it is probable that, as I 
then observed in a footnote, this relation was obscured by 
the situation of the spicules in different planes of the body- 
wall—this factor in spicule development defeating comparison. 
With regard to the spicular sponge-work composing the 
oral plates of Thyone [variety (d) above] many scleroblasts 
can be observed adhering to the various parts of the reticulum, 
but no relation can be traced between any particular local 
conformation of the skeleton and the presence of a scleroblast. 
This sponge-work doubtless arises in the same manner as the 
similar sponge work of Asteroids, Echinoids, and Ophiuroids 
arises, viz. by the fusion of primitively separate elements, 
each of which is equivalent to the individual plate-spicule of 
Cucumaria or Thyone, and therefore probably develops in the 
same manner, 1. e. as regards the number of scleroblasts, ete. 
I have myself observed the perforated plates of Amphiura 
elegans and of the recently-metamorphosed Hchinus 
esculentus prepared by the picro-carmine and _ lichtgriin 
method, and the number of scleroblasts attached to them 
amply confirms this supposition. On the other hand, the 
lateral spines of many Ophiuroids and the axial skeleton of 
Crinoid arms, judging merely from observation of the adult 
structures, appear to develop in a manner different from that 
just stated. Whether such is the case, however, I hope to 
ascertain at some future date. : 
In conclusion I wish to express my thanks to Dr. H. J. 
Allen, who kindly afforded me every facility during my visit 
to the Plymouth Marine Biological Laboratory in 1904, in 
obtaining material, and to Prof. Minchin, who originally sug- 
gested to me that I should turn my attention to holothurian 
spicules, 
