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THE VOYAGE OF H.M.S. CHALLENGER. 
surface of the domes on either side, then a lateral horizontal growth takes place, till the 
cladal margin is exactly adapted to the curved sides of the adjacent dome. So, too, 
should a cladus bifurcate at a little distance from an opposing dome lying directly in its 
course, the angle of bifurcation becomes filled up by a horizontal growth till the margin 
of the dome is reached. And again, should three domes, occupying the angles between 
the protocladi of a spicule, lie near enough to the centre of the cladome, then these angles 
also become filled up, and the cladome assumes a plate-like form, with curvilinear notches, 
corresponding to the curved sides of the domes. Thus, with regard to the poral domes, 
the tendency of the cladi is to grow out into flat plates extending up to them. This 
tendency is disturbed by the interference of the spicules one with another. If a growing 
cladus encounters the rhabdome of a spicule belonging to the series next above it, it 
bifurcates, the deuterocladi passing on each side of the rhabdome, or it curves round it 
without bifurcating. If a cladus, after bifurcating to adapt its outer margin to the 
outlines of two domes which lie behind and on each side of the point of bifurcation, 
encounter the cladus of another spicule at right angles, the angle of bifurcation becomes 
filled up by lateral growth, and a triangular plate results, the distal base of which is 
accurately adapted to the side of the opposing cladus. 
This freedom and adaptability on the part of the cladome is in striking contrast 
to the rigidity of the rhabdome of these spicules, to the rigidity of the cladi of the 
homologous ectosomal spicules of some other Lithistids, and of all Choristids, and, in a 
word, to the rigidity of all spicules, traversed throughout by a continuous axial rod. The 
submissiveness of the cladi of the spicule to the influence of the environment is therefore 
in all probability correlated with the suppression of the clad -axial fibres, which, as we have 
already seen, extend but an insignificant distance from their origin, while the axial fibre 
of the rhabdome extends from origin to end. 
Genus 2. Discodermia, Bocage. 
Tetracladidse in which some of the ectosomal megascleres are discotrisenes, and the 
microscleres are microxeas and microstrongyles; with differentiated oscular and poral sur- 
faces ; the pores are simple, singly distributed, the oscules are numerous and simple. 
Discodermia discifurca, n. sp. (PI. XXXII. figs. 1-11). 
Sponge (PL XXXII. fig. 1). — Irregularly cup-shaped ; two or more cups borne on a 
stout pedicel, which ends below in a flat, somewhat expanded, attached base. Walls of 
the cup thick, cavity shallow, margins rounded. Large oval openings on the exterior 
lead into winding tubular involutions of the external surface. Oscules confined to the 
inner surface of the cup ; pores single, irregularly dispersed over the external surface. 
