ECOLOGY AND TAXONOMY OF Halimeda 63 



To reflect these differences he proposed a division of the Siphonales 

 into two new orders, the Eusiphonales (a name which was also untenable) 

 and the Caulerpales. Taxa of the Eusiphonales were homoplastic and 

 had a wall containing cellulose ; those of the Caulerpales were hetero- 

 plastic, and lacked cellulose in their walls. The division into families, 

 and the genera assigned to them are included in Table VII which shows 

 this subdivision of Siphonales. 



Feldmann (1954) developed this yet further when, on the basis of 

 available life-history data, he divided his two orders of 1946 (Eusi- 

 phonales and Caulerpales) each into two more orders (Table VIII). In 

 doing so he abandoned the epithet Eusiphonales, thereby removing 

 one nomenclatural problem. 



From the Eusiphonales he separated the orders Derbesiales and 

 Codiales. The Derbesiales were established as possessing a hetero- 

 morphic life-cycle, or a life-cycle in which the alternating gametophytic 

 and sporophytic phases looked very different, and as a consequence 

 may have been given different names. The culture studies of Kornmann 

 (1938) provided some of the basic data, for Kornmann had observed the 

 zoospores of Derhesia marina (Lyngbye) Sol., a small, branched fila- 

 mentous alga, develop into plants identifiable as the sac-like Halicystis 

 ovalis (Lyngbye) Aresch. And Feldmann (1950), in the years subsequent 

 to his 1946 paper on siphonalean taxonomy, had linked Derhesia 

 tenuissima (De Not.) Crn. with Halicystis parvula Schmitz. 



In contrast, members of the Codiales were known for a life-cycle 

 in which there was only one free-living phase (haplobiontic) which was 

 diploid in these algae. As another difference between the two new 

 orders special gametangia developed on the filaments of members of the 

 Codiales, whereas specialized structures did not occur in the Derbesiales, 

 the entire thallus of the gamete-bearing phase, Derhesia, functioning as 

 a gametangium. 



The other two orders of Feldmann's (1954) paper were derived by 

 separating the oogamous Dichotomosiphon tuherosus (A. Br.) Ernst, a 

 freshwater filamentous alga, from the Caulerpales, and placing it in its 

 own order because of this distinctive reproductive feature which is 

 unknown in the rest of the Siphonales. In contrast, the Caulerpales, 

 from the data then available on the life-histories of Caulerpa and 

 Halimeda, produced gametes that were only slightly different (aniso- 

 gamous). In addition, the entire contents of the thalli of these two 

 genera were transformed into gametes when gametogenesis occurred, 

 so that death of the plant followed sexual reproduction (holocarpy). 



In both of the Feldmann (1946, 1954) schemes the classification of 

 Halimeda is in the family Udoteaceae of the order Caulerpales. 



