486 W. WOODLAND. 



naturally a far greater amount of material had to be examined) 

 excepting the initial granule. I may here remark that the 

 numerous specimens labelled ''Synapta digitata" which 

 have been forwarded to me differ considerably in their 

 spiculation — in the forms and sizes of the spicules — and I 

 cannot help suspecting that the S. digitata examined by 

 me includes several distinct species, or at least sub-species. 

 The specimens of S. inhcerens and S. hispid a, on the 

 other hand, have always been constant in their spicule 

 characters. In most specimens of S. digitata the spicules 

 differ considerably in size and form in the same animal — a 

 feature I have not noticed in the other two species. 



The Development of the Spicule in Synapta inh^kens. 



The first sign of the future spicule is the multiplication of 

 the cells of the dermal or outer layer at one point. I say 

 ''cells," but, strictly speaking, it is a multiplication of nuclei 

 to form a syncytium, since cell-outlines are rarely, if ever, 

 distinguishable. This preliminary formation of a syncytium 

 is best seen in a section of the body-wall (fig. 1). It will be 

 observed in tig. 1 that in these young Synaptas the dermal 

 layer is already quite separate from the circular muscle layer, 

 the intervening space containing, at this stage, irregular 

 strands of nerve- and muscle-fibres. These initial syncytia 

 are easily distinguishable in section from the numerous 

 thickenings of the dermal layer which form the sense-organs, 

 since the elongated cell-outlines and pigment deposits are 

 conspicuous features in these latter. It is also easy to dis- 

 tinguish them in surface view under a low magnification after 

 a little practice. The sense-organ thickenings are, needless 

 to say, much the more numerous. 



The actual spicule first appears as a small spherical granule 

 situated in the centre of the syncytium, but more internally 

 than externally, so that the majority of the nuclei are situated 

 on the other side of the spicule when this is viewed from the 

 internal side of the body-wall (fig. 2). This spherical granule 



