REPORT ON THE TETRACTINELLIDA. 
421 
and this will lead to the overgrowth of those spicules which have become the “ hispi- 
dating ” spicules of the sponge ; in the Tethyidse the growth has been of a different 
character : the original Ehagon has increased from the centre radially, and in accordance 
the megascleres lie with their long axis in the same direction. Both arrangements 
are readily derivable from that which occurs in the simpler Tetillidse, and is repre- 
sented diagrammatically in Fig. 4, a. In this case the spicules are arranged both 
concentrically and radially ; let the thin-walled vasiform Epallax arise from this in the 
manner described above, and the concentrically arranged spicules will become the chief 
or axial spicules of the skeleton, as in Fig. 4, b ; if, on the other hand, the walls increase 
in thickness, as in Tethya and Craniella, then the radial spicules as in Fig. 4, c, form 
the main and indeed only megascleral skeleton. In the Tetillidse we can trace the 
Fig. 4. — A, Primitive arrangement of spicules met with in Tetilla. B, arrangement in Epallax. C, Tethya and 
Craniella j k, concentrically, r, radially arranged spicules. 
transition from the stage shown by A to that of c in Fig. 4, but in the Tethyidse the 
earlier stages are lost or remain to be discovered, and only the final term is known 
to us. 
In the Scolopidse we meet with the palisade arrangement of the cortical oxeas that 
is already foreshadowed in Placinastrella, the only Tetractinellid sponge in which it is 
so well expressed, though something similar is presented by the cortical oxeas of some 
of the Geodiidse ; this arrangement persists throughout the Suberitidse, which probably 
originally branched off from some point near the root of the Scolopidse. 
