66 



PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



[Dec. 4, 



radii are normally of the same length ; and consequently the centres 

 of these hexaradiate elements are all equally distant (y^nr") from the 

 centre of the element they surround. Thus each hexaradiate element 

 is continuous with six other hexaradiate elements symmetrically 

 arranged around it ; and these six elements are similarly continuous 

 with other elements surrounding them in turn, and so on till the 

 limiting surfaces of the organism are reached. Thus a vertical 

 radius is continued upwards as a vertical fibre, and a horizontal 

 radius is similarly continued as a horizontal fibre ; and in this way 

 all the radii fall into three groups of more or less parallel fibres : 

 (i.) vertical or longitudinal fibres, composed of a series of vertical 

 radii, radiate from the base of the ventriculite upwards, diverging 

 as they go in gentle curves to the outer wall; (ii. and iii.) horizontal 

 radii in two series, one group concentric with the cloaca of the 

 sponge, and the other radiating from it. The centres of the hexa- 

 radiate elements are the points where the fibres of these three series 

 intersect. 



This typical arrangement of the elements is subject to very 

 numerous variations. 



(i.) One ray from an element, instead of diverging from its fellows 

 at right angles, bends away some 45° from its normal course to fuse 

 with the ■similarly inclined radius of an adjoining element (fig. 4). 



Fig. 3. 



Fig. 4. 



(ii.) Two parallel rays from a pair of elements combine with two 

 other rays, furnished not from two other elements, as they should be 

 normally, but from a single element alone. By these abnormalities 

 (i. and ii.) the radii join to form a triangle instead of the ordinary 

 rectangle. In this way is produced a change of direction in the 

 course of the fibres. 



(iii.) In a series of pairs of elements, the fibres between the 

 centres of succeeding elements gradually become longer, till, at the 

 third or fourth pair from the point at which the increase in length 

 commenced, an additional element is introduced, to preserve the nor- 

 mal distance between the diverging series. 



