REPORT ON THE TETRACTINELLIDA. 
13 
Sometimes, again, and this is a fact of some importance, a cladus of either a protrisene or 
anatrisene gives off a secondary cladus at the point of enforced bending, aud thus becomes 
dichocladose. I have previously suggested that bendingdeads to the budding of fresh cladi 
at the point of flexion, and this seems to be a case in point ; for dichotrisenes are unknown 
amongst the Tetillidse as. regular constituents of the skeleton, and we have not the least 
fragment of evidence to show that they have descended from ancestral sponges which once 
possessed them, but many general considerations to the contrary. Hence, when bifurcation 
arises, as in this case, we must regard it as a variation appearing de novo, and may attempt 
to account for it by the conditions of the case, which appear in this instance to be the 
action of secondary pressures or tensions different to those under which the regular 
trimne form was produced. 
The oxea also exhibits interesting departures from the normal type. The distal end is 
not unfrequently rounded off, and this may occur close to where the point would other- 
wise have been, or a millimeter or more away from it. Not only so, but secondary cladi 
may be developed from it, thus producing in a rudimentary and unsymmetrical form 
a kind of prodisene spicule (PI. V. fig. 13). 
The young anatrisenes (PI. V. fig. 7) differ from the adult in that they terminate 
distally in a swollen bulb-like end, through which the axial rod or fibre passes, and after 
enlarging somew^hat in the middle of the cladome, continues right up to the end of the 
spicule. From the sides of the bulb the cladi project, making a larger angle with the 
rhabdome than they do in the adult spicule. In the adult spicule the axial fibre is not 
continued past the origin of the axial fibre of the cladi. In the young forms one some- 
times meets with a projection of the rhabdome for some considerable distance beyond 
the cladome ; it then terminates in a rounded end (PI. V. figs. 8,9). This seems to me 
a point of some significance in connection with the question of the origin of the triaene. 
The differentiation of the anatriaenes into those of the cortex and those of the roots, 
seems to stand in connection with the additional tension to which those of the roots are 
exposed. 
The young protriaenes (PL V. figs. 10, 11) differ from the adult in somewhat the 
same fashion as in the case of the anatriaenes. 
Tetilla grandis, var. alha, nov, (PI. V. fig. 3). 
Sponge similar to Tetilla grandis, but distinguished by the absence of an anchoring 
basal mass. In addition, the flagellated chambers are larger, attaining a diameter of 
from 0'0513 to 0’0671 mm. 
The spicules, which are quite similar to those of Tetilla grandis, gave the following 
measurements; — Oxea 5‘7 by 0’063 mm.; protrisene 11‘78 by 0‘0316 mm.; trichodal 
protrisene I'O mm. long; anatrisene 19 "6 mm. long, 0‘0118 mm. broad in the middle 
