ox MERLIA XOin[AM. 687 



the gaps left by the dissolved bars of the calcareous skeleton. 

 In this situation a few calcocytes are seen in the walls of the 

 arches. 



(5) Theoey of Construction of the Calcakeods Skeleton. 



The skeleton is formed by the granular amoebocTtes or 

 calcocytes. The youngest stages of the skeleton (Pi. 35, figs. 

 20, 21) consist simply of microscopic scales, flakes or lumps 

 laid down on the surface of the shell or other body which 

 Merlia incrusts. 



Within the growing edge of a fla*-, thin, spread-out specimen 

 of the sponge the edge of the skeleton is visible in the form 

 of slender bars foi-ming a poly'gonal network, the polygons 

 being incomplete at the extreme edge. In a still younger 

 stage, visible only under fairly high powers, the slender bars 

 are no more than smears or streaks of flakes, scales or lumps, 

 somewhat higher at points where the tubercles will be formed. 

 In course of growth the smears become ridges, and the poly- 

 gonal outlines grow into circular pits with a rim of tubercles^ 

 and finally the pits become tubes with tabulas varying in 

 number from one to five, but commonly with one or two. 



I l)elieve that this complicated structure can be accounted 

 for in the following way : Merlia is a thin incrusting- 

 siliceous sponge which has acquired the character of forming a 

 calcareous skeleton. This sponge, like many other Tetraxonid 

 sponges, is a modification of the simple "rhagon," i.e. of a 

 thin-walled flattei:ed sack with the choanosomal folds branch- 

 ing out all round. The mode of growth is that of dichotomous 

 branchlets extending out from centre to periphery and possibly^ 

 anastomosing with or in close juxtaposition to neighbouring" 

 branches (see radiating spokes on PI. 33, fig. 1). At some 

 epoch in its history — a point to be discussed later — the meta- 

 bolic cells, the amcebocy^tes, became fed up with carbonate of 

 lime, and either underwent calcification or re-secreted the lime. 

 These cells would necessarily occupy the spaces in the sponge 

 between the choanosomal branches, especially at points of 



