STUDIES ON THE COMPAEATIVE ANATOMY OF SPONGES. 7 



16), and extremely inequiradiate and inequiangular spicules, 

 of the form termed by Haeckel (7) '' sagittal/^ on the other. 

 In the sagittal spicules two rays are about equal in length, 

 while the third has either grown out into a very long and 

 slender shaft, which may attain to three or four times the 

 length of either of the others (figs. 13, 14), or (much more 

 rarely) remained short while the other two have grown long 

 (fig. 18). The angle between the two paired arras, or the oral 

 angle, as Haeckel terms it, is greater than either of the other 

 two angles, which are equal. 



Figs. 13 to 19 represent seven triradiate spicules, and 

 illustrate the variation in the proportions of the rays and 

 angles. The commonest form is the sagittal, with one ray 

 (the shaft) much longer than the other two. It is important 

 to notice that, as a general rule at any rate, the three rays of 

 the triradiate do not all lie in exactly the same plane, so that 

 if the spicule were laid down upon an even surface it would 

 rest upon the ends of the three rays, with the centre elevated. 

 This fact is not shown in the figures, which are merely out- 

 lines drawn with the camera. In the sagittal spicules the 

 shaft is usually perfectly straight, but often of the beautiful 

 spear-like form shown in figs. 13 and 14. The two paired 

 rays, on the other hand, are often slightly curved. 



The triradiates vary considerably in size. The following 

 measurements are taken from a well-grown spicule, of the 

 form shown in fig. 13, and all the other spicules (from figs. 

 7 to 20 inclusive) are drawn to the same scale. 



Length of the shaft 0'38 mm. 

 Length of the paired rays 0"12 mm. 



Quadriradiate Spicules. 



These are probably to be regarded only as further modifi- 

 cations of the fundamental triradiate type, for although they 

 are quadriradiate they are still only triaxial. Their shape is 

 usually that represented in fig. 20. It will be seen that the 

 third ray of a normal triradiate spicule has become much 



