194 



DESM1DIA CE^E. PART n. 



the circulating fluid flows round the obtuse end, but 

 the latter gradually assumes the form of the elongated 

 end, the regular circulation of the fluid is established, 

 and in five or six hours after the separation, two young 

 desmids are formed precisely similar to their parent, 

 the Closterium Lunula. 



The Cosmarium, another Desmid, consists of a cell of 

 two lobes united by a narrow isthmus. When about to 

 multiply, the isthmus swells into two globular expan- 

 sions, separated from each other and from the two lobes 

 of the cell, by a narrow neck. These enlargements in- 

 crease and assume the appearance of half segments of 

 the original cell. In this state the plant consists of four 

 segments lying end to end, the two old ones forming 

 the extremes, with the two new ones in the middle. 

 At last, each of the middle segments gets a new half, 

 which soon acquires the full size and characteristics 

 of the old one. This process, which is accomplished in 

 twenty-four hours, is repeated ere long, and being con- 

 tinued indefinitely, the extreme lobes of the row are 

 thrust farther and farther asunder, and the whole con- 

 stricted thread or chain of Cosmaria is enclosed in a 

 gelatinous sheath. The last two central lobes contain 

 no portion of the original frond or plant, and may thus 

 be considered to be entirely new individuals. 



Many of the Desmidiacese multiply by the subdivision 

 of their endochrome into a multitude of granular par- 

 ticles called gonidia, which are set free by the rupture 

 of the cell wall, and of which every one may develop 

 itself into a new cell. The gonidia may be zoospores 

 with cilia and active locomotion, or they may be en- 

 closed in a firm envelope, and become resting spores. 

 The movement of the zoospores at first within the 

 cavity of the cell which gave them their origin, and 

 afterwards externally to it, has frequently been ob- 

 served in the varied species of the genus Cosmarium, 



