382 Scientific Proceedings, Royal Dublin Society. 



plane of its surface. This never happens with any of the trira- 

 diates of Grantia ciliata and Grantia conipressa ; but in a Leuconia, 

 L. hotryoides, which I have examined, many of the triradiates do 

 fulfil this condition, remaining constantly extinguished as they lie 

 flat on the microscope slide, between crossed nicols. In these re- 

 gular triradiate forms the optic axis passes then through the centre 

 of the spicule perpendicular to its plane. 



This is not the case, however, with all, or indeed the greater 

 part of, the triradiates of this species ; most of them behave in the 

 same manner as the sagittal and all the other triradiates of Gran- 

 tia conipressa and G. ciliata; these appear brilliantly illuminated 

 between crossed nicols ; but on rotation they extinguish in four 

 positions at right angles to each other. In these positions it will 

 always be found that one ray of the triradiate lies parallel to the 

 long diagonal of one nicol, i.e. in the plane of its principal section. 

 With the sagittal spicules it is the unpaired ray that has this posi- 

 tion ; with the regular and sub-regular triradiates, it may be any 

 one of the three rays. In the same way the ouadriradiates extin- 

 guish ; and in their case it is always the apical ray which lies in 

 the principal plane of the nicol at the time of extinction. The 

 optic axis must also lie in this plane, and thus the morphological 

 axis of the unpaired ray and the optic axis of the sagittal spicule 

 are in the same plane which is at right angles to the plane of the 

 spicule (fig. 3). 



Fig. 3. 



Longitudinal vertical section through the middle of an unpaired ray of a sagittal spicule, showing 

 the position of a principal section, o d a d' \ of the optic axis, o a; and an axis of maximum 

 elasticity, e e' . A ray of light passing perpendicularly through the spicule is shown by r r . 



The spicules behave as single individuals, aU parts becoming 

 extinguished and illuminated together ; so that while the optic axis 



