ZYGNEMA. 135 



10. ZYGNEMA ^7. 



Char. Endochrome arranged in spiral order within each cell. 

 Sporangia generally oval, and never lodged in the trans- 

 verse tubes of communication. 



Derivation. From wyo, a yoke, and vrjjaa, a thread. 

 Spirogyra Link, Handb. iii. 261., Meyen, in Linna3a, ii. 

 410. Choaspis Gray. Salinacis Bory, in Diet. Class. 

 xv. p. 75. c. iv. t. 45. Conjugates sp. Vauch, Conf. d'Eau 

 douce. 



The first of these genera is characterized by the endo- 

 chrome with its brilliant granules (which Miiller, in his sur- 

 prise on first discovering a species of the genus, likened to 

 precious stones), being arranged in a spiral form, the number 

 of spires being from one to eight. 



The species of this genus admit readily of division into two 

 subgenera, in the one of which the filaments unite, and in 

 the other no conjugation takes place ; and each of these di- 

 visions allows of further analysis, founded on the conforma- 

 tion of the cells. 



" In the first of these subdivisions, which for the most 

 part includes the long-celled species of the genus, such as 

 Zygnema elongatum and Z. quadratum, &c., the opposed ex- 

 tremities of all those cells which have attained maturity are 

 considerably inverted, and which inversion may be compared 

 to that of the finger of a glove (PI. xvn. fig. 4.) ; while in the 

 second, which embraces the short-celled examples, as Zyg- 

 nema maximum, Z. nitidum, and very many others, the cells 

 are not inverted, but touch each other by their plane surfaces. 



" The form of this inversion is, in all the species in which 

 it occurs, identical and extremely regular, its circumference 

 being circular, and its base somewhat flat ; no membrane in- 

 tervenes between the spaces formed by this indoubling in 

 contiguous cells, which spaces therefore communicate directly 

 with each other. 



" At the period of reproduction, and at no other, .one of 

 the two indented and opposed extremities of certain cells be- 



K. 4 



