64 L. HILLIS-COLINVAUX 



3. Caulerpales: its classification among the Chlorophyta, or Chloro- 

 phycophyta {Papenfuss, 1946) 



(a) Round's scheme of classification. Relatively recently Round 

 (1963, 1971) considered the classification of the Chlorophycophyta as a 

 group, and his 1971 scheme is shown in Fig. 21. Only that part which 

 impinges on Halimeda will be discussed. 



The old order Siphonales is replaced by five orders, four of which 

 are those of Feldmann (1954). In the fifth, Phyllosiphonales, Round 

 places the single genus Phyllosiphon, which, as he suggests, may belong 

 to the Xanthophyceae. Ostreobium, which is commonly allied with 

 Phyllosiphon and which would seem reasonable to include here, is 

 mentioned with the Chlorochytriales. Families within these orders are 

 not discussed in these papers. 



Round (1963, 1971) places the siphonaceous orders in the separate 

 class Bryopsidophyceae, and within the class recognizes the three 

 groups or "cohorts" as he calls them, and which appropriately could 

 be considered subclasses, of Hemisiphoniidae, Cystosiphoniidae and 

 Eusiphoniidae, which were set up by Chadefaud (1960) as Hemi- 

 siphonees, Eusiphonees cystosiphonees and Eusiphonees ty piques. The 

 orders within each are shown in Fig. 21. The Hemisiphoniidae, with 

 walls dividing the thallus into multinucleate units, is the simplest of the 

 three groups. Members of the Cystosiphoniidae produce cysts and may 

 or may not have segregative division. Taxa belonging to Eusiphoniidae 

 possess the additional carotenoid pigments siphonein and siphonoxan- 

 thin, have polysaccharides other than cellulose in their cell walls, and do 

 not have the special characteristics of the other two groups. 



(i) Advantages of Round's scheme. For siphonaceous algae the 

 system of cohorts (subclasses) brings together three series of green algae 

 with the multinucleate condition. The designation of a separate class 

 (Bryopsidophyceae) for this group of three recognizes some of their 

 distinctive differences from the other green algae, and seems particularly 

 reasonable in a system which recognizes a number of classes of green 

 algae (Zygemaphyceae, Oedogoniophyceae and Chlorophyceae). There 

 are, as Round (1971) summarizes, a number of important features in the 

 Bryopsidophyceae such as pigments, wall chemistry and plastid 

 structure which separate them from the class Chlorophyceae, but none 

 of these embraces the class as a whole. 



(b) Classification scheme of Bold and Wynne (1978). In their massive 

 and splendidly comprehensive introductory text on the algae these two 

 authors recognize 15 orders within the division Chlorophycophyta 

 (Table VI). One of these, the Caulerpales, replaces the former 



