50 W. WOODLAND. 



under undisturbed conditions is liere clearly demonstrated ; 

 there is in these aplacophore spicules no question whatever 

 as to whether or no the symmetrical form is due to crystal- 

 lisation or the like. And, bearing- this example in mind, it 

 further cannot be denied that the straightest rays of the 

 triradiates of clathriuid Calcarea in all probability owe their 

 symmetry to the same cause. Crystalline matter is deposited 

 by the terminal cell in a similar manner in both cases, and, 

 so long as the conditions remain undisturbed, the growth of 

 the spicule must continue in a straight line. " Bio-crystal- 

 lisation'^ and the rest are here at least superfluous. On the 

 other hand, introduce disturbed conditions and, as might be 

 expected, the more irregular spicules of Leucosoleniidfe and 

 Sycons are the result. Further, granting placid conditions, 

 bring three cells into such close apposition that their inner 

 surfaces become adpressed into the outline of a triradiate 

 and let each of these cells divide centripetally, the distal cell 

 in each case producing, in relation with the fixed proximal 

 cell, a straight monaxou,^ and personally I can see no reason 

 why cumbrous hypotheses should be invented in order to 

 explain why the three contained angles of such a triradiate 

 spicule (found in most calcarea) should in almost all cases be 

 equal. Surely the equiaugularity is the direct result of the 

 apposition of the three scleroblasts, as I have previously 

 contended [10]. 



No such explanations, however, apply to the young tri- 

 radiate '^stars'' of most echinoderms described in the last 

 Study, which, however, differ, like most other spicules, from 

 the calcareous spicules of sponges and Aplacophora in that 

 they are entirely invested by the formative protoplasm. 



The Spicules in the Ascidian genus Leptoclinum. 



I have examined three species of Leptoclinum — L. com- 

 mune, L. maculosum, and L. sp. — all obtained from 



* This monaxon is, under these conditions, pointed at both ends. In 

 Aplacophora eacli spicule, corresponding to its one-celied basal origin, is 

 truncated at its proximal end. 



