510 GILBERT 0. BOURNE. 



only say with certainty that the spicule is composed of a 

 number of very minute fibro-crystals. The organic matrix 

 left after decalcification consists of a close feltwork of ex- 

 ceedingly fine fibres, the whole enclosed in a distinct and 

 apparently structureless spicule sheath. The similar spicules 

 of Xenia and Heteroxenia, which I have described in 

 another place (4), behave under polarised light in exactly the 

 same way as those of Clavularia coerulea, and their organic 

 basis is also similar. 



Secondly, there are the flat scale-like spicules which occur 

 in Primnoa, Plumarella, and in many other Alcyonarians. 

 Examination by polarised light shows that the scale-like 

 spicule of Priranoa and Plumarella is made up of a 

 number of crystals of calcium carbonate lying in nearly the 

 same plane, and radiating from a common centre. Fig. 6 

 is a drawing of a single spicule of Plumarella delicatis- 

 sima, Wright and Studer, and fig. 7 is a drawing on a 

 smaller scale of the same spicule when viewed through the 

 polariscope with the prisms uncrossed. The characteristic black 

 cross leaves no doubt as to the crystalline arrangement of 

 the spicule, and exactly the same effect is obtained with many 

 inorganic crystalline deposits, e. g. with platino-cyanide of 

 magnesium. So far as I have been able to determine, these 

 scale-like spicules in Primnoa and Plumarella are formed 

 by several cells, or at least by a comparatively large coeno- 

 cytial investment containing many nuclei. The material at 

 my disposal is not sufficiently well preserved to allow me 

 to speak with certainty on this point, but sections made 

 through the zooids of both genera show that the scale-like 

 spicules, which form opercula corresponding in number to the 

 tentacles, and situated at the bases of these organs, are each 

 enclosed in a cavity in the mesogloea, which cavity is lined by 

 a protoplasmic layer in which many nuclei are embedded. The 

 specimens were too much macerated by the prolonged action of 

 spirit to enable me to say definitely that no cell outlines are 

 present in the protoplasmic lining, but at all events I was un- 

 able to discover any in sections stained by various methods and 



