296 ALG^ 



Literature. 

 Ehrenberg — Die Infusionsthierchen, 1838. 

 Busk — Trans. Micros. Soc, 1853, p. 31. 

 Cohn— Jahrber. Schles. Gesell., 1856, p. 237 ; and Beitrage, i., 1875, Heft 3, p. 93 



(Pop. Sc. Rev., 1878, p. 225). 

 Carter— (Eudorina) Ann. Nat. Hist., 1858, p. 237 ; and 1859, p. i. 

 Braun— Bot. Zeit., 1875, p. 189. 



Stein — Der Organismus der Infusionsthiere, part iii. , 1878. 

 Henneguy— Journ. d. Micrographie, 1878, p. 485. 

 Maupas — Compt. Rend., Ixxxix., 1879, p. 129. 

 Kirchner in Cohn's Beitrage, iii., 1879. 



Order 2. — Hydrodictye^. 



The relationship of the Hydrodictyese to the other famiUes of Coeno- 

 bieae is somewhat obscure. They differ from them in the form of the 

 coenobe, which, instead of being minute and globular, ellipsoidal or tabu- 

 lar, is of considerable size, and presents the appearance of a net. The 

 only known mode of reproduction is by the conjugation of zoogametes. 



As here constituted the order is limited to the single genus Hydro- 

 dictyon Roth, one species of which is moderately common in ponds 

 and ditches, and is known under the name of ' water-net.' When the 

 plant is mature, the coenobe consists of a sac-like net several centimetres 

 in length, composed of a great number of cyhndrical cells united at their 

 ends so as to form a 4- or 6-cornered mesh. The ordinary mode of pro- 

 pagation consists in the protoplasm of one of the cells breaking up into 

 from 7,000 to 20,000 tnegazoospores, each furnished with four cilia, which 

 move about with a trembling, motion within the zoosporange, come to 

 rest in the course of half an hour, and then arrange themselves in such 

 a way that, by their elongation, they again form a net of the original 

 kind, which is set free by the absorption of the wall of the mother-cell, 

 and attains, in the course of three or four weeks, the size of the mother- 

 colony. In other cells of the mature net the protoplasm breaks up into 

 from 30,000 to 40,000 smaller swarm-cells or zoogametes, furnished 

 with only two cilia, which at once leave the gametange and swarm 

 about for some hours. Conjugation between these has been observed to 

 take place even within the gametange, more than two sometimes uniting 

 together. The resulting zygosperm retains its green colour, and invests 

 itself with a firm cell-wall. After remaining for several months at rest 

 as a hypnosperm, it begins to grow slowly, and, when it has attained 

 a considerable size, its contents break up into two or four mega- 

 zoospores, which come to rest after a few minutes, and assume a pecu- 

 liar angular form when they have attained a considerable size, putting 

 out horn-like appendages. In each of these polyhedra the green parietal 



