36 W. WOODLAND. 



diameter. Most of the plates in tlie young Ophiothrix are 

 extremely thick, and with comparatively few and small per- 

 forations (fig. 11) ; however, the ordinary thinner plates are 

 also present. The spicule originates in the same manner as 

 that already described (figs. 8 — 10). The young forms 

 represented by figs. 10 h and c are^ as already mentioned, 

 unusual. Fig. 10 c indeed reminds one as regards shape of 

 a youDg stage in the formation of an auricularian wheel. 

 Fig. 13 represents a young spine which develops in much the 

 same way as the stool of Thyone, save of course that the 

 basal plate originates as a granule in one cell, and that the 

 number of scleroblasts concerned in its deposition is much 

 greater. It is quite possible that not all the cells repre- 

 sented in this figure are scleroblasts, some possibly belonging 

 to the surrounding membrane, and it is clearly impossible to 

 distinguish between them ; it is quite safe to say, however, 

 that most of them are scleroblasts. 



In Echinus esculentus (and miliaris) also the deve- 

 lopment of the spicule follows on the same lines (figs. 14 — 18). 



The Plate-spicules in Ante don bifida. 



The majority of the spicules situated in the region of the 

 disc of Antedou lie towards the inner side of the soft integu- 

 ment, and are of two kinds — the imperforate thin ''glas- 

 plattchen " of comparatively wide diameter and concentric- 

 ally marked (see accompanying text-figure), and the ordinary 

 perforated plates, which, however, often assume a very irre- 

 gular shape, and occasionally become mere branching struc- 

 tures. All transitions (fig. 21) are to be found between the 

 " glasplattchen,^' which are very unechinoderm in appear- 

 ance, and the ordinary perforated plates ; the former, how- 

 ever, are much the more common, and exist in great inimbers. 

 Since the development of the perforate plates and bi-anching 

 spicules is quite normal, i.e. the same as that already 

 described for Amphiura, Ophiothrix, and Echinus, I shall 

 merely describe the simple development of the imperforate 



