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-