116 Blackman and Tansley. 
Genus. 
Derbesia. Sober, 1847 . 
Characters of the family. 
[Though the vegetative characters of this genus suggest a position 
among the lower Siphoneae in the neighbourhood of Bryopsis, yet the 
peculiar zoospores, recalling those of CEdogoninni, probably indicate 
that its affinities are widely different. Our present information is not 
however sufficient to justify the removal of the genus from its 
traditional place.] 
Fam. IV. CAULERPACli^E. 
Thallus a branched coenocyte; axis a creeping, branching 
“rhizome” bearing fixing, slightly branched “roots” below, and 
“fronds” above. The latter are very variously differentiated, often 
closely imitating the external forms of different higher plants. Vacuolar 
cavity traversed by “beams” of cellulose, whose function is 
probably to tie the external walls together. 
Reproduction by fragmentation of the thallus; the wounds so 
produced being very readily healed. No reproductive cells known. 
Genus. 
Caulerpa. Lamouroux, 1809 . 
Characters of family. The genus is divided into twelve 
sections according to external characters. 
Fam. V. Codiace^e. 
Thallus a much branched ccenocytic tube. Branches thread-like, 
loosely or closely interwoven to form a thallus of definite external form, 
which may shew an external differentiation of parts, and an internal 
differentiation into a central region with longitudinally running threads 
(pith), and a peripheral region with outwardly directed branches (cortex). 
Thallus fixed below by root-threads. Surface of thallus often wholly 
or partially incrusted with calcium carbonate. Chromatophores 
numerous, round or elliptical, with or without pyrenoids. 
Reproduction insufficiently known; sometimes by the formation 
of biflagellate swarmers in special branches (zoosporangia or game- 
tangia) shut off from the main tube ; usually by fragmentation (?) 
Genera. 
1 . Avrainvillea. Decaisne, 1842 . 
Thallus either undifferentiated, of loosely interwoven, 
dichotomously branched threads, or differentiated into 
a stipe and frond, with the constituent threads closely 
interwoven to form a compact tissue-like body. No 
differentiation into pith and cortex. 
2 . Penicillus. Lamarck, 1813 . 
Thallus consisting of a cylindrical or slightly flattened 
stipe formed of closely interwoven threads and 
encrusted with calcium carbonate, terminated by a 
crown of free, diverging, dichotomously branched 
threads. Root of numerous dichotomously branched 
threads. Stipe with well marked differentiation into 
pith and cortex. 
