ECOLOGY AND TAXONOMY OF Holimeda 



191 



Halimeda. An electron microscope study of calcification in Halimeda 

 (Wilbur et al., 1969), however, showed that the pattern was different. 

 Working with cultured incrassata and monile, Wilbur et al. (1969) 

 noted that aragonite crystals were first formed on the outer surface of 

 the utricle walls when the segment was about 36 hours old (Fig. 55), 

 although this varies somewhat with the material, and that the crystals 

 formed in the immediate vicinity of the fine fibrils of the filament wall. 

 No organic matrix was observed at the site of crystallization (Fig. 58) 



Fig. 58. H. monile. An early stage in the development of aragonite crystals. Both 

 granular crystals and needles occur in the region of the fibres of the filament wall. 

 The needles extend at various angles into the interutricular spaces. Scale bar is 

 0-1 [iva. 



and in contrast to other calcification systems it also was not evident in 

 sections decalcificated with uranyl acetate which retains an organic 

 matrix in molluscan shell. Crystal orientation was random. These 

 aspects have subsequently been confirmed with cylindracea (Borowitzka 

 and Larkum, 1977). 



Sections through older material showed that following the formation 

 of crystals within the fibrous matting of the walls of the peripheral 



