474 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



edges or borders of the partitions, and here only, that skeletal 



formation occurs (giving rise to the 

 netted shell with its hexagonal meshes 

 of Fig. 221), so also at times, in the 

 case of such little aggregates of cells 

 or vesicles as the four-celled system 

 of Callimitra, it may happen that 

 about the external hoinidary -lines, 

 and not in the interior boundary- 

 planes, the whole of the skeletal 



Fig. 228. An isolated portion of matter is aggregated. In Fig. 228 we 



the skeleton of Dictyocha. . . 



see a curious little skeletal struc- 

 ture or complex spicule, whose conformation is easily accounted 

 for after this fashion. Little spicules such as this form 

 isolated portions of the skeleton in the genus Dictyocha, and 

 occur scattered over the spherical surface of the organism 



Fig. 229. Dictyocha stapedia, Hkl. 



(Fig. 229). The more or less basket-shaped spicule has evidently 

 been developed about a little cluster of four cells or vesicles, 

 lying in or on the plane of the surface of the organism, and there- 

 fore arranged, not in the tetrahedral form of Callimitra, but in 

 the manner in which four contiguous cells lying side by side 

 normally set themselves, like the four cells of a segmenting egg: 

 that is to say with an intervening " polar furrow," whose ends mark 

 the meeting place, at equal angles, of the cells in groups of three. 

 The little projecting spokes, or spikes, which are set normally 

 to the main basket-work, seem to be incompleted portions of 

 a larger basket, or in other words imperfectly formed elements 

 corresponding to the interfacial contacts in the surrounding parts 



