THALLOPHYTA. 645 



rdle, having fallen to a subordinate position. Only in forms like Vaucheria clavata, 

 adapted to life in rapidly-flowing water, have the zoospores a primary importance. 

 Sexual reproduction is here difficult, and zoospores are always produced in abun- 

 dance on the slightest change of conditions. 



Bryopsidacece. This family consists of marine, mostly tropical, forms. The 

 plant-body of Bryopsis has quite a definite form, and consists of a tube forming the 

 main axis, fixed below by short root branches, and bearing above in acropetal suc- 

 cession a series of branches, some of unlimited, some of limited growth. In these 

 latter are formed swarming cells which are of two sizes, the smaller being yellowish, 

 while the larger have each a green chromatophore. It seems very probable that 

 these are anisogametes, but their conjugation has not been observed. 



Derbesia is a genus like Bryopsis in many respects, but with special zoosporangia 

 which produce curious zoospores, each with an anterior crown of cilia. 



Caulerpacece. The genus Caulerpa contains nearly one hundred species, which 

 present the most varied external forms, simulating those of many of the higher 

 plants, such as Mosses, Ferns, Mare's-tails, Cactuses, Conifers, &c. Each plant, how- 

 ever, consists simply of a single much-branched but uninterrupted tube, the branches 

 taking the forms of roots, leafy shoots, &c. The tube is supported internally by a 

 complicated system of " beams" of cellulose which run out from the walls. 



The Caulerpas live mainly in tropical and subtropical seas. They often grow 

 together in large masses, forming great beds of sea-weed, their creeping stems or 

 " rhizomes" extending many yards. No reproductive cells have as yet been found 

 in any of them, multiplication taking place apparently solely by the breaking off of 

 parts of the thallus, which drift and fix themselves elsewhere. 



CodiacecB. Under this name we may conveniently place together a group of 

 forms specially characterized by a thallus consisting of richly-branched tubes, which 

 are interwoven to form a mass of more or less solid character, which possesses in 

 each genus a definite and characteristic external conformation. Thus, Penicillus has 

 a long cylindrical "stalk" fixed below by "rhizoids" and bearing above a head of 

 free dichotomously branching radiating filaments. The older parts of the stalk are 

 strongly incrusted with calcium carbonate. 



Udotea has a stalk often creeping and branching, bearing flat fan-shaped 

 fronds. Spherical bodies, the nature of which is unknown, are borne on short side 

 branches of the tubes of which the frond is built up. Halimeda possesses a thallus 

 mainly composed of series of heart- or kidney-shaped segments, which give many of 

 the species the appearance of an Opuntia. There is usually a considerable deposit 

 of calcium carbonate covering the thallus. Roundish structures, produced in grape- 

 like bunches on the edges of the segments, liberate swarming cells whose behaviour 

 has not been followed. 



Codium has no well differentiated stalk or segments; the thallus is very various 

 in form, and is differentiated into a well-marked pith and cortex, the tubes being 

 mainly longitudinal and loosely packed in the former, while the latter consists of 

 club-shaped closely-packed branches arranged at right angles to the surface. In 



