STUDIES IN SPICULE FOKMATION. 43 



Tig. 2. — The origin of tlie plate-spicule in Amphiura elegans as a 

 spherical granule contained within a single scleroblast. 



Fig. 3. — The triangular and subsequently triradiate forms assumed by the 

 initial granule within the scleroblast. 



Fig. 4. — Young (usually) trifid spicules with two scleroblasts attached 

 (derived from division of the mother scleroblast). 



Fig. 5. — Older stage of the triradiate. In a one nucleus is dividing; in b 

 one nucleus has divided, three scleroblasts being present. 



Fig. 6. — The bifurcation of the arms of the triradiate. The number of 

 scleroblasts has increased, and nuclei are seen to be about to divide. 



Fig. 7. — Older spicules with numerous scleroblasts. 



Figs. 8 — 12 represent stages in the formation of the usually thick plates 

 (with small perforations) of Ophiothrix fragilis. In Fig. 10, b and c 

 represent young stages of unusual shape. Fig. 12 is a young spicule of the 

 thin plate variety. 



Fig. 13 represents one of the lateral trifid spines of 0. fragilis. The 

 numerous scleroblasts in connection with the spicule is striking when com- 

 pared with the two scleroblasts associated with the somewhat similar "stool" 

 spicule of Thyone (Study IV, pi. 34). 



Figs. 14—18 represent young stages in the formation of the plates in 

 Echinus esculentus similar to those above. 



PLATE 4. 



Development of the imperforate plate-spicules of Antedon bifida (Figs. 

 19 — 22), and of the plate-spicules of Synapta hispida (Figs. 23 — 29), and 

 S. digitata (Fig. 30). 



Figs. 19 — 22. — The development of the imperforate plate-spicule of 

 Antedon bifida. Fig. 21 represents spicules which show some affinity in 

 their form to the ordinary perforate plate-spicules of echinoderms. The size 

 attained by the adult thin perforate plate may be realised by comparing the 

 present figures of young plates (x 640) with the text-figure showing adult 

 plates (x 270). 



Figs. 23—30 represent the development of the curious plate-spicules o 

 Synapta hispida and S. digitata, which are very small in comparison 

 with the plate-and-anchor spicules. Fig. 24, representing a young stage in 

 the development of the plates of S. hispida, shows forms very similar to 

 those of Fig. 30, which represents adult spicules of S. digitata. Figs. 

 25 — 28 illustrate the enclosure of the nucleus within the central perforation. 

 Fig. 29 shows plates in which the central aperture is considerably larger than 

 the average. 



