560 E. A. MINCHIN. 



honeycomb over again. I can understand, however, that an 

 attempt might be made to explain this characteristic regularity 

 in a different manner. It might be urged, since each spicule 

 is formed by three cells of more or less equal size, closely 

 pressed together, that therefore the rays would naturally meet at 

 angles of 120° as the result of their being formed in the axis of 

 the cell ; but after carefully weighing this theory I am obliged 

 to reject it as an adequate explanation, for the following 

 reasons. First, because the actinoblasts are never exactly equal 

 or perfectly regularly placed, nor are the rays formed exactly 

 in the axis of the cell, but almost always a little to one side or 

 the other j hence if that were the only factor at work, we 

 should rather expect irregularity to be the rule, and equality 

 between the angles to be a rare exception. Secondly, any such 

 explanation, even if accepted for the spicules of the Clathri- 

 nidse, which have the three rays meeting at equal angles, 

 would fail completely in Leucosolenise, where we find indeed 

 great regularity, but of a different type. In Leucosolenia 

 the triradiate systems are of a pronounced bilateral type, with 

 two curved lateral rays, each meeting the straight posterior ray 

 at an angle much less than 120°, and enclosing anteriorly an 

 angle much greater. We may express it by saying that while 

 eacVi lateral angle is, in a projection (120 — w)°, the anterior 

 angle is {120 + 2n)°. Now if the orientation of the rays is to 

 be referred to the arrangement of the actinoblasts, the bilateral 

 form of the triradiate systems of Leucosolenia could only be 

 explained by a corresponding arrangement or difference in size 

 of the actinoblasts, which would bring us to an explanation 

 which would itself require to be explained.^ 



I cannot, therefore, regard the regularity of the triradiate 

 systems as due to the arrangement of the secreting cells. On 

 the other hand, the view that this, their most striking charac- 

 teristic, arises from adaptation to the structural requirements 



* So also Ludwig, discussing the calcareous bodies in the Holothurians, 

 refers the branching at angles of 120° to the arrangement of the cells 

 (" Echinodermen," I, in ' Bronn's Thierreich,' ii, 3, p. 60). But what ex- 

 plains the arrangement of the cells ? 



