452 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



spores of a certain little group of parasitic protozoa^ the Actino- 

 myxidia. These spores are formed from clusters of six cells, 

 of which three come to constitute the capsule of the spore; and 

 this capsule, always triradiate in its symmetry, is in some species 

 drawn out into long rays, of which one constitutes a straight 

 central axis, while the others, coming off from it at equal angles, 

 are recurved in wide circular arcs. The account given of the 

 development of this structure by its discoverers* is somewhat 

 obscure to me, but I think that, on physical grounds, there can 

 be no doubt whatever that the quadriradiate capsule has been 

 somehow modelled upon a group of three surrounding cells, its 

 axis lying between the three, and its three radial arcs occupying 

 the furrows between adjacent pairs. 



Pig. 217. Spicules of hexactinellicl sponges. (After F. E. Schultze.) 



The typically six-rayed siHceous spicules of the hexactinellid 

 sponges, while they are perhaps the most regular and beautifully 

 formed spicules to be found within the entire group, have been 

 found very difl&cult to explain, and Dreyer has confessed his 

 complete inability to account for their conformation. But, 

 though it is doubtless only throwing the difl&culty a little further 

 back, we may so far account for them by considering that the 

 cells or vesicles by which they are conformed are not arranged in 



* Leger, Stole and others, in Doflein's Lehrbuch d. Protozoenkunde, 1911, 

 p. 912. 



