INTRODUCTION. 29 



curved, are observed in the soft parts of the young lips of 

 shells, both marine and freshwater." Several of each of these 

 organs may be found in each cell. 



All the separate and distinct structures and parts entering 

 into the formation of the Confervas have now been treated of: 

 certain peculiar adaptations and contrivances still remain to be 

 described, such as the inversion of the extremities of the cells 

 in many Zygnemata, the corrugated cells of the Vinculifera, 

 the layer of fibres surrounding the main stems of some 

 Batrachosperms, the ciliary processes met with in the same 

 genus and others nearly allied, and the presence of silex in 

 some Diatomacece. The further notice of these, however, will 

 be postponed until the individual genera come under con- 

 sideration. 



The rapidity of the growth of Confervas, and indeed of all 

 the articulated Algce, has often been a subject of surprise to 

 many observers of Nature ; and to none more than myself has 

 it occasioned greater astonishment, until I became acquainted 

 with the reason of so surprising a developement of structure. 

 If the filaments of Zygnema nitidum be carefully examined 

 and contrasted together, it will be seen that in some the 

 length of the cells only just exceeds their diameter, and that 

 each cell usually contains four spiral coils, which together 

 perform from seven to eight turns in each, the coils almost 

 touching each other : that in other filaments the length of the 

 cells is more than three times the diameter, but that still each 

 cell contains only the same number of revolutions of the 

 spires, viz. seven or eight, which now, instead of being nearly 

 in opposition, are widely separated ; thus plainly proving the 

 elongated cells to be derived from the extension of the 

 shorter ones. And again it will be noticed in other filaments, 

 that the cells have returned to their original length, but that 

 each now contains only three or four spiral turns, thus 

 affording manifest proof of the division of the elongated 

 cells, and completing the chain of evidence which establishes 

 to demonstration the existence of the modes of growth to 

 which I have referred throughout all the cells in the species 

 of the genus Zygnema. The proofs now to be adduced, that 



