728 Mr. Edward A. Minchin [May 6, 



chimneys. In this peculiar growth two distinct types are found. 

 In one type (Clatlirina) the tubes form a close network opening by 

 a few short oscula, usually very inconspicuous. In the other type 

 (Leucosolenia) the sponge has a more erect form and consists chiefly 

 of the conspicuous chimneys, united by an inconspicuous network of 

 small tubes. Now in the former type of architecture the pressures 

 and strains in the network of tubes are dilferent at different spots, 

 and cannot be said to predominate in one direction more than another. 

 Hence, in ClatLrina, we might expect to find a type of spicule adapted 

 to these conditions, and as a matter of fact, the predominant spicule 

 here is the triradiate with equal rays and equal angles : that is to say, 

 an evenly balanced form fitted to resist tensions in any direction 

 equally. But occasionally a Clathrina grows in a more erect and 

 stalked form, and then strains in a vertical direction predominate ; 

 in such a case (e.g. CI. hlanca, CI. lacunosa) the arm of the spicule, 

 which is placed vertically, becomes greatly strengthened, especially 

 in certain regions, the other two arms remaining small, sometimes very 

 much so. In all cases, however, the equal angles are still retained. 



In Leucosolenia, on the other hand, the erect growth requires 

 strengthening chiefly in a vertical direction, and the form of the 

 triradiate spicule is at once seen to correspond with this, having 

 paired angles and a form which at once suggests adaptation to pressure 

 in one direction rather than another. The spicules are placed with 

 great regularity, the unpaired ray directed vertically, and the paired 

 rays horizontally, so that the whole forms a beautiful basket-work, 

 stiffened by vertical ribs and held together by horizontal girders. It 

 is thus seen that even subordinate peculiarities of form have their 

 special uses, which are evident when studied in connection with the 

 architectural requirements of the whole organism. 



The result, therefore, of an inquiry into the relations between 

 the living crystals and the organism by which they are formed, is 

 as follows: that both in their form and arrangement the spicules 

 represent a most exquisite piece of engineering, and are to be re- 

 garded as adapted to support and protect the fragile and delicate 

 body wall. Moreover, the history which has been traced for the 

 development of the spicules is shown to be not altogether imaginary 

 by the facts of the development of the spicules, which may now be 

 briefly considered. 



The calcareous spicules are formed within cells, derived from the 

 external layer of the body wall, but each ray or branch owes its 

 origin to a distinct cell. In the simplest case one cell forms a single 

 rod-like spicule, and when a very large rod is to be formed, the 

 mother cell may multiply into two or more daughter cells. When 

 a triradiate is to be formed, three mother cells come together, one 

 for each ray, and after each has divided into two daughter cells, they 

 secrete three separate rods, which sooner or later become joined to- 

 gether to form the spicule. When a quadriradiate is to be formed, 



