236 W. WOODLAND. 
symmetrical disposition in the region of the oscular rim, i.e. 
above the region of the lateral chambers. This regular and 
symmetrical disposition of the triradiate spicule consists of 
one ray being vertical in position and situated next the base 
of the sponge, and the two companion rays in consequence 
(since the spicule is equiangular) lying towards the apex of 
the sponge at an inclination of 30° to the horizontal. The 
reason for this regular disposition of the triradiates will, as 
also in the case of the monaxons, be supplied later. 
Triradiates situated in the “body” of the sponge are 
approximately both equiangular and equiradiate in form, but, 
corresponding to the unlike conditions to which they are sub- 
ject, those situated in the extreme upper and lower regions 
of the sponge depart somewhat from this type. ‘l'riradiates, 
e.g. of adult Sycons situated in the oscular rim, have their 
paired rays the more depressed towards the horizontal the 
nearer they are situated to the upper extremity (text-fig. 1), 
and (to provide an explanation which will be more fully 
appreciated when Part Il has been read) it seems probable 
that this depression is associated with the greater tendency 
of the wall to be invaginated in this region (see p. 265)—just 
as the upraised arms of a semaphore apparatus, joined hori- 
zontally by a spring, would incline the more to the horizontal 
the greater a weight borne by them. Generally speaking 
the “structural differentiation of the rays (in sagittal triradi- 
ates) is correlated with their position and function in the 
sponge” (Minchin'), and, as is doubtless the case elsewhere, 
the secondary forms just mentioned are, in all probability, 
determined in each individual instance by exposure of the 
formative (apical) cells, during their activity to the incident 
forces peculiar to the localisation of the spicule in the sponge 
—formative cells naturally being susceptible to all such influ- 
ences. Indeed, the conformation of these apically-situated 
triradiates proves this susceptibility of the formative cell, for, 
at their central junction, the three rays are strictly equiangular, 
i. e. contain three equal angles—showing that the spicule would 
1 Lankester’s ‘* Treatise on Zoology ;”” Chapter on Porifera. 
