234 Siphon ales 



septum with a central aperture, and in Codium and other genera the septa 

 are frequently complete, forming ' stoppers ' in various parts of the thallus. 



The attachment of the Codiacea? to the substratum is effected by 

 numerous, repeatedly-branched rhizoids, which often penetrate deeply into a 

 soft substratum, or fix themselves firmly to sand-grains, fragments of shells, 

 remains of calcareous Alga?, etc. Some species, such as Udotea Desfontainei 

 and Halimeda Tuna, only occur attached to stones or to the rhizomes of 

 Zostera. The rhizoids possess fairly thick walls, but only a delicate proto- 

 plasmic lining and very few chloroplasts. 



In the branches which form the ' shoot ' the lining layer of protoplasm is 

 much thicker, containing numerous discoidal chloroplasts of a rounded or 

 polygonal form, and with or without a pyrenoid. Very often the apical parts 

 of the branches are of a deeper green colour owing to the aggregation of the 

 chloroplasts at these points. Numerous starch-grains are present, especially 

 in the older parts of the filaments, and in outward shape they may be 

 reniform, fusiform, or irregularly ovoid. In Avrainvillea a yellow or brown 

 pigment may be present in the cytoplasm. 



The Codiacea? are largely propagated by fragmentation of the thallus, 

 which often occurs by the formation of numerous new shoots, the older parts 

 dying away and thus setting free the new shoots as young plants. At other 

 times new plants are developed from the merest fragments, even from a 

 single 'palisade' branch of the old thallus (Tobler, 11). Zoogonidangia and 

 gametangia are developed in some genera on the sides of the peripheral (or 

 palisade) branches. The reproductive cells are biciliated and the gametes 

 are anisogamous (fig. 155 D G). 



It has been found that the Codiacea? are best and most naturally 

 separated into three sub-families, the Flabellariea?, the Udotea?, and the 

 Codiea?, of which the two first-named are more nearly related to each other, 

 than either group is to the Codiea?. This classification was proposed by 

 A. & E. S. Gepp ('11), who regard the Flabellariea? and Udotea? as funda- 

 mentally distinct, the separation having probably occurred far back in their 

 phylogenetic history. 



Sub-family FLABELLARIE^E. This is the largest section of the Codiacea?, 

 including nine genera all of which are destitute of calcification. The thallus 

 shows considerable diversity of form in the different genera, although the 

 general tendency is towards the formation of a stalked and flattened expan- 

 sion. Chlorodesmis and Rhipidodesmis are the most primitive genera, the 

 former affording some clue to the ancestry of Flabellaria and probably of the 

 flabelliform genera Avrainvillea, Rhipiliopsis, Rhipilia, and Cladocephalus. 

 The largest genus is Avrainvillea in which the thallus consists of dicho- 

 tomously branched, interwoven filaments forming subsessile or stalked, 



