Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/110

66 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 4, radii are normally of the same length; and consequently the centres of these hexaradiate elements are all equally distant (1/100") 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 hexaradiate 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).

(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 normal distance between the diverging series.