488 W. WOODLAND. 



the shaft. A number of these (usually from sis to ten), which 

 up to the present have been situated either at the sides of the 

 shaft or on its external aspect, now migrate on to its internal 

 side, and form a small cluster in that position (fig. 12). This 

 fact seems to me to be most remarkable and I am quite 

 unable to account for it, at least in an adequate manner. 

 This cluster of nuclei is always easily distinguishable from 

 the rest of the syncj^tium, and it is this specialised part which 

 gives rise to the plate spicule. Thus, although the anchor 

 and plate portions of the entire aggregate spicule are quite 

 separate, and without doubt equivalent to two spicule 

 individuals — to two echinoderm plate-spicules, to be precise — 

 yet they are both developed from the same syncytium, though 

 in distinct parts of it. 



The plate-spicule, like other spicules, first originates as a 

 granule, and this is situated in the centre of the internal 

 cluster of nuclei (fig. 13). This granule next elongates on 

 opposite sides and at right angles to the length of the 

 anchor-shaft (cf . the development of the plate-and-anchor 

 spicule of Synapta digitata described below) to form a 

 rod or rhabdus (figs. 14, 15, 16) which is at first pointed at 

 both ends. Tlie nuclei of the cluster are situated on both 

 sides of this rod, and in fact the entire development of the 

 plate spicule is identical with that of the plate spicules of the 

 Cucumaridfe described by me in Study IV, with the excep- 

 tion that the number of nuclei initially concerned is greater 

 in the present instance. The rod thickens at its extremities 

 (fig. 16), then bifurcates (fig. 18), these primary bifurcations 

 elongate and themselves bifurcate (fig. 20), and the processes 

 of bifurcation and fusion of the extremities continue (figs. 21, 

 22), as in the Cucumariau plate-spicule, until the adult plate 

 is produced (fig. 24). The Synapta plate-spicule is obviously 

 different in several respects from the Cucumariau plate- 

 spicule, but the plan of structure is the same. The Synapta 

 plate possesses extremely large perforations, Avhich diminish 

 in size towards the more pointed " basal " extremity, and the 

 edges of these perforations develop small processes which 



