238 W. WOODLAND. 
cated in text-fig. 2), i.e. the posterior rays are here shorter 
for the same reason that the terminal segments of a crab’s 
claw are shorter than the more proximal segments, or caudal 
vertebre than thoracic. ‘Towards the base of the sponge, 
however, actual flexion is very small, and hence there is more 
room for elongation of the posterior rays; moreover, this 
elongation is here probably aided by the fact that the stresses 
due to flexion of the sponge are greatest in this region, and 
are, of course, borne entirely by the longitudinal element of 
the skeleton; in other words, it is quite possible that this 
mechanical stimulation exerts an influence on the formative 
cells, leading to their increased activity. Additional evidence 
TextT-Fic. 2. 
of the foregoing explanation as to the cause of elongation of 
the posterior rays of basal triradiates is afforded in the case 
of other sponges. In the erect Leucosoleniidz, and in those 
forms of Clathrinide ‘‘ characterised by a more erect growth, 
such as Clathrina blanca and lacunosa, the posterior ray 
is indicated by its greater length, so that the triradiates 
become sagittal. In lacunosa this feature is carried to an 
extreme in the stalk, where a distinct peduncular skeleton is 
developed, composed partly of sagittal triradiates, partly of 
dinctinal monaxons,”’ Minchin). ‘The triradiates in this last 
instance are of the “tuning-fork” type, and afford a good 
illustration of growth taking place in the direction of least 
resistance (in the ovoid body of lacunosa supported by the 
