1898.] on Living Crystals. 725 



crystals, but also that each one is a single crystal, a fact discovered 

 independently by Sollas and Ebner. Their crystalline nature is 

 shown both by their beLaviour to polarised light and by etching 

 experiments. They do not answer to the cleavage test so satisfac- 

 torily, probably on account of the organic matter with which their 

 substance is interpenetrated. But other tests show them to be true 

 calcite crystals, distinguished, how^ever, by a peculiar form, which 

 can best be illustrated by imagining each spicule to have been, as it 

 were, cut by a lapidary out of a single block of crystal^ just as a 

 diamond is cut into a faceted form which is not that of the natural 

 diamond crystal. This comparison must only be taken as an illusti-a- 

 tion, however, and not as a description of how the spicule is formed, 

 for it is not carved out of a block, but is built up to its shape, just as 

 a stone house is not hewn out of solid stone, but built up of separate 

 stones. 



It is seen that the great difference between the living and the 

 lifeless crystal is one of external form. In view of the regularity 

 and symmetry of the calcite crystal, and the very precise geometrical 

 laws that govern its form, the differences in this respect exhibited 

 by the living crystal become very striking. It is evident that some 

 disturbing influence must be at work which interferes with the natural 

 development of the crystal. We know that if a calcite crystal deve- 

 lops of itself, it assumes a certain form. In order to discover what 

 has caused the living crystal to take on its curious and unusual 

 growth, we must examine the conditions under which it has arisen. 

 Hence it is now necessary to leave for a moment the crystalline aspect 

 of these spicules and look at them from another j)oint of view, as 

 portions of a living body. To do this we must understand something 

 of the animal which has produced them and the part which they play 

 in its internal economy. 



The simplest calcareous sponge or Olynthus is an organism very 

 easy to understand. It can be compared to a thin-walled vase, with 

 a wide opening at the top, and a great many minute openings or pores 

 on the sides. During life an internal mechanism produces a current 

 of water which flows in through the pcres into the cavity and passes 

 out by the opening or osculum at the summit. All calcareous sponges 

 start life in this condition, and the form and structure, whatever it 

 may be, which they have when full grown depends simply on the 

 manner in which the Olynthus grows. Hence this organism may be 

 considered as representing probably the primitive type of sponge 

 which was the ancestor of the whole group, and which is not found 

 anywhere at the present day as an adult form, but occurs always in 

 the life-history as a transitory stage, in which the structure of the 

 sponge is found reduced to its simplest terms. 



Now the wall of the young sponge is very thin and delicate, and 

 could not support itself were it not for the spicules which stiffen it. 

 When the body wall is examined more closely it is seen that the 



Vol. XV. (No. 92.) 3 b 



