138 CONJUGATE^;. 



tKem having nothing whatever to do with the separation of 

 the cells, and never being in any case the result of it, but 

 depending, as explained already, upon unequal internal pres- 

 sure, and occurring chiefly at the period of reproduction. 

 The effect of the eversion is, as already observed, to occasion 

 the dislocation of the cells. 



: " Again, in every filament of those Zygncmata which ex- 

 hibit the inverted structure, cells may be observed terminating 

 in the ordinary manner of Confervoe, viz. by plane surfaces, 

 the presence or absence of the inversion depending upon the 

 period of the formation of the disse2:>iments ; the older ones, 

 or, as observed in the beginning of tliis notice, the more 

 mature ones only presenting it. Thus it follows that the 

 opposed extremities of cells always exhibit the same structure, 

 and that this alternation in form supplies evidence the most 

 conclusive of the multiplication of cells throughout the eyitire 

 Jilament of a Conferva by division." * 



, It is in this genus that the radiated organ described in the 

 Introduction is best developed, and may be observed with 

 most advantage ; it is here also that the cruciform organs, 

 sup^Dosed to secrete the raphides, and the curved strings of 

 cytoblasts, before noticed, are most clearly seen. 



Attention to the structure of the cells in the Zygnemata is 

 of the very first importance in the discnmination of species, 

 it being the only distinguishing character in some closely 

 allied species. 



First Subgenus. — Filaments conjugating. 

 * Extremities of the cells truncated. 



a. Spires numerous. 



1. Zygnema orbiculaee Hass. 



Plate XIX. Figs. 1, 2. 



Char. Filaments highly mucous, and of a light green colour, 



* From a paper entitled " Observations on some Points in the Ana- 

 tomy and Physiology of the Freshwater Algse," by Arthur II. Hassall, 

 inserted in the "Annals and Mag. of Natural History," vol.xii. pp. 26 — 8. 



