ACHLYA; HYDRODICTYON ' 565 



quite mature, they are set free by the rupture of its wall (B), and, 

 after swarming about for a time, develop into tubiform cells resem- 

 bling those from which they sprang. Each of these zoospores is 

 possessed of two flagella ; their movements are not so powerful as 

 those of the zoospores of Vaucheria, and come to an end sooner. 

 The generative process in this type is performed in a manner that 

 may be regarded as an advance upon ordinary conjugation. The 

 end of one of the long tubiform cells enlarges into a globular dilata- 

 tion, the cavity of which becomes shut off by a transverse partition. 

 its contained endoplasm divides into two, three, or four segments, 

 each of which takes a globular form, and is then fertilised by the 

 penetration of an antheridial tube which conies off from the filament 

 a little below the partition. The oospores thus produced, escaping 

 from the globular cavities, acquire firm envelopes, and may remain 

 unchanged for a long time even in water, when no appropriate nidus 

 exists for them ; but will quickly germinate if a dead insect or other 

 suitable object be thrown in. 



One of the most curious forms of the lower algje is the ' water- 

 net,' Hydrodictyon reticulatum, which is found in fresh- water pools 

 in the midland and southern counties of England. Its frond con- 

 sists of a green open network of filaments, acquiring, when full 

 grown, a length of from four to six inches, and composed of a vast 

 number of cylindrical tubular cells, which attain the length of four 

 lines 01- more, and adhere to each other by their rounded extremi- 

 ties, the points of junction corresponding to the knots or intersections 

 of the network. Each of these cells may form within itself an 

 enormous multitude (from 7,000 to 20,000) of zoospores, which at a 

 certain stage of their development are observed in active motion in 

 its interior, but come to rest in the course of about 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 

 dissolution of the wall of the mother-cell, and attains in the course 

 of three or four weeks the size of the mother-colony. Besides these 

 bodies, however, certain cells produce from 30,000 to 100,000 

 ' microzoospores ' of longer shape, each furnished with four long 

 flagella and a red ' eye-spot ; ' these escape from the cell in a swarm, 

 and move freely in the water for some time. Conjugation between 

 these smaller zoospores has been observed to take place sometimes 

 even with the mother-cell. The resulting body or ' zygospore ' 

 retains its green colour, but becomes invested with a firm cell- wall 

 of cellulose. In this condition these bodies may remain dormant 

 for a considerable time, and are described as ' hypiiospores ' or 

 ' resting-spores ; ' and in this state they are able to endure being 

 completely dried up without the loss of their vitality, provided that 

 they are secluded from the action of light, which causes them to 

 wither and die. In this state they bear a strong resemblance to the 

 cells of Protococcus. The first change that manifests itself in them, 

 when they begin to germinate, is a simple enlargement ; next the 

 endochrome divides itself successively into distinct masses, usually 

 two or four in number ; and these, when set free by the giving way 

 of the enveloping membrane, present the characters of ordinary 



