^82 K. KIRK PATRICK. 



they form larg-e flat plates with the nuclear region projecting 

 out from the free surface (PL 'SO, figs, 6, 7, 8). 



JW'^hatever their disguise, they possess certain common 

 chai-acters which enable them easily to be recognised. They 

 are crammed with large orange-coloured pigment-granules, 

 the colour beiug retained for a loug time in specimens pre- 

 served in gh'cerine or formalin. Further, they have a large, 

 soft, spheroidal or oval, vesicular nucleus and a distinct 

 nucleolus. The granules, which stain easily and deeply, vary 

 apparently according to the state of the metabolism — that is 

 to sa}', in the early stages in the history of a calcocyte the 

 granules are often of unequal size, but when they are in a 

 position to lay down lime they become more or less uniform. 



When the granules are pressed out of a cell and viewed 

 under a high power slightly out of focus they appear as 

 uniformly light discs, and gradually, as they are brought 

 into focus, a dark point appears in the centre surrounded by 

 a light circle ; on focussing down further the dark point 

 enlarges till the circle becomes uniformly dark. The appear- 

 ance of the dark point in the light circle is of some importance, 

 because it is seen all over the surfaces of the calcareous 

 skeleton, where the ends of the fibrillse give this appearance, 

 and even in the interior of the conules on the tubercles (PI. 

 35, fig. 22, and PI. o6, fig. 9). The granules appear to be the 

 essential calcigenous elements. 



AVherever the skeleton is being laid down these granular 

 calcocytes abound, and in decalcified specimens they are 

 found in the closest relation witli the organic basis of the 

 dissolved skeleton. 



PI. 34, fig. 3, shows a decalcified tubercle and side of a 

 polygonal mesh with the calcocytes closely applied. 



Fig. 1 on PI. 37, which represents the extreme growing 

 edge of the sponge, is especially interesting, for it shows 

 the calcocytes planning out the first foundations of a polygonal 

 mesh on the surface ot" the shell on which the Merlia is 

 growing. The calcareous products of calcocytes in this 

 position are flakes or scales (see Section 2b, and PI. 35, fig. 21). 



