CALCAREA. 7 



Spicules (Fig. 13). 



The following description is due to Professor Minchin : — 



Oxea. — The monaxon spicules of this species are very constant and characteristic. 

 They are divisible at the outset into (a) ordinary and (6) refringent monaxons : — 



(a) The ordinary monaxons are curved and vary greatly in dimensions, from 

 about 100 /A to 450 m or more in length, and from 3/u, to 16 m in thickness. 

 Every possible gradation is to be found between the greatest extremes in 

 size, so that it is not possible to divide these spicules into different classes. 

 The shaft is thickest near the proximal end, which is also nearly straight 

 for rather more than half the length; from this point the shaft curves 

 evenly, and tapers slightly, towards the distinct lance-head, which usually 

 shows very plainly the form of a double bend, especially in the smaller 

 examples of this type of spicule. Among the ordinary monaxons there are 

 always a very few V type monaxons : these are very rare, and only one or 

 two are usually found in a whole spicule slide. 



(6) The refringent monaxons are much scarcer than the ordinary type, but they 

 are easily found, as their optical peculiarity makes it easy to distinguish 

 them from the others, especially under a low power of the microscope. 

 Like the ordinary monaxons, they show a wide range of variation in size, 

 from small to very large. Their form is similar to that of the ordinary 

 monaxons in a general way, but shows certain constant peculiarities ; the 

 shaft is very slightly curved, sometimes nearly perfectly straight ; the 

 proximal swelling of the shaft contrasts more sharply with the distal 

 extremity, which is often very slender and tapers down rapidly about half- 

 way from the proximal end ; and the lance-head tends to be rudimentary 

 or even quite absent. By these special features the spicules can easily be 

 recognised as a distinct class, apart from their characteristic appearance 

 under the microscope. 



Triradiates. 



(c) The ordinary triradiates are of fairly large size, in form very similar to 

 those of Leucosolenia complicata, with the unpaired rays markedly longer 

 than the paired rays. The unpaired ray is straight and slender, varying in 

 length from about 1 10 ju, to as much as 185 /a. The paired rays are generally 

 slightly thicker than the unpaired, sometimes distinctly so ; they curve 

 symmetrically, first in a backward direction for the proximal two-thirds of 

 their length, then forwards for the distal third ; in length they vary from 

 lOOju, to 145/*, the average length being about mid- way between these two 

 extremes. The anterior angle is an open one, but the lateral angles are 

 always greater than right angles, the spicules never being T-shaped as in 



VOL. IV. 2 I 



