Ixxxii 
THE VOYAGE OF H.M.S. CHALLENGER. 
the cladi of a protrisene, while the deuterocladi are almost invariably tangential in 
position. 
The anatrisenes present a difficulty which is not readily overcome : this is the 
recurvature of the cladi. In the very young spicule they are extended from the tylus 
at right angles to the rhabdome, that is in an exactly tangential direction, but with 
growth they begin to curve backwards till they may become almost parallel to the 
rhabdome. Hitherto we have not been careful to distinguish between a line of least 
resistance due to direction of growth and one due to tension, for either would in most 
cases serve our purpose equally well, but they are evidently of different character, for 
a line of growth is to be represented by a vector, while a tension if represented 
vectorially must be drawn as two equal and opposite vectors ; it is possible that this 
distinction may be of service to us here, for if the anatrisene be compared with the 
other forms of this trisene, it will be found that it is generally, I think always, 
distinguished by a longer and thinner rhabdome ; this points to a relatively quicker 
growth in a radial direction, and owing to this the cladome of the spicule will encounter 
resistance as it is pushed forwards, if so the cladi, which from their inception point 
outwards at right angles to the rhabdome, will grow in backwards, as this is the direction 
of least resistance. Of course, there are difficulties in the way of this explanation, but 
a complete theory of spicules is not evolved all at once. One difficulty is involved in 
the often excessive length of the rhabdome, a character we have employed in our 
explanation, for it might fairly be argued from the line of reasoning here followed that 
this excessive length is itself a result of tension, and just what we should expect from 
the function of this spicule, which is partly to resist the pull of currents on the sponge 
when it lives anchored by these spicules in the ooze of the sea floor. 
If therefore the rhabdome is produced by a vis d f route acting at the cladal end, how 
can a vis d ter go be called in to account for the backward direction of the cladi ? 
As an interesting example of the influence of tension on spicules, we may refer to 
the appearance of accessory cladi, which now and then make their appearance in triaene 
spicules ; thus beneath the cortex of some sponges are found large, somewhat spherical, 
cavities, which have already been alluded to as subcortical crypts, and it sometimes 
happens that spicules passing by the sides of the crypts develop a lateral spur, such 
as is shown in PI. XLIII. fig. 19 at <x. This is clearly due to the tension exerted by 
the membrane of the roof of the crypt on the adjacent tissue, a tension which was not 
sufficient to divert the rhabdome from its course, but was sufficient to elicit an 
accessory cladus, and this has just that direction it should have according to theory. 
A curious point of detail represented in the figure just referred to may be noticed in 
passing ; it is the total disconnection of the axial fibre of the accessory cladus and that of 
the rest of the spicule. I have observed one or two other instances of this, but they are 
not common. ' 
