ECOLOGY AND TAXONOMY OF HciUmeda 



I Enlarged in next figure 



CORTEX r PRIMARY 1- 



3 series of utricles { SECONDARY 2- 

 (_ TERTIARY 3 



s flown 



MEDULLA 



with medullary 

 filaments 



INTERFILAMENTAL SPACES 

 exterior to utricles(filaments) 

 where calcification begins — 



NODE where next segment will grow 



Fig. 6. Longitudinal sections through part of a segment (incrassata) showing its basic 

 filamentous construction. Note absence of cross walls throughout, which makes this 

 plant a coenoeyte. In the right-hand diagram the medullary filaments at the ajDex 

 of the segment (right) have started to grow and branch to form a new segment. 

 Such filaments show a distinct gradient in kinds and quantities of organelles. 

 (Redrawn by D. Dennis, The Ohio State University, from Hillis (1959).) 



SEAWATER 



Primary Utricle 



— Interf ilament Space 

 Secondary Utricle — 



Tertiary Utricle 



CORTEX 



MEDULLA 



Fig. 7. Schematic enlargement of a small portion of the cortex illustrated in Fig. 6 

 showing variable thickening of filament wall at bases of utricles, and the mterfila- 

 mental spaces of the cortex. Those of the medulla are not included. It is in these 

 interfilamental spaces that aragonite is deposited. (Drawing by D. Dennis, The 

 Ohio State University.) 



absent in the juncture between segments. Consequently, the inter- 

 segmental region or node, for a very short distance, is composed only of 

 axial filaments. For most species this is where the filaments fuse in 

 some characteristic pattern, a circumstance useful to taxonomy 



Fig. 5. (OpjDosite.) H. cylindracea, a non-decalcified jjlant from Enewetak lagoon 

 showing the usual appeai-ance of the holdfast. The holdfast shown was one of the 

 smaller ones of over 50 specimens examined from the lagoon. 



