OSCILLATORIACEjE. 85 



cially when newly made, forming a greenish-black 

 closely adherent stratum. Under the microscope it is 

 seen to consist of innumerable palish-green filaments ; 

 these are jointed or transversely striated, some being 

 straight, others curved, the ends often exhibiting a 

 writhing or worm-like movement. The appearance 

 of these fibres is peculiar, seeming as if they were 

 solid throughout, and so differing from that of the 

 Confervacese, in which the cell-walls are readily dis- 

 tinguishable from the cell- contents. The fibres easily 

 break across at the joints ; and the last few segments 

 are often narrowed and rounded, so as to form a blunt 

 point. When they have been left in water, they ex- 

 hibit colourless tubular sheaths surrounding and 

 extending beyond them. These sheaths consist of 

 the consolidated outer portions of the cell- walls ; for 

 when the cells undergo transverse division, and ex- 

 pand by growth in the direction of the length of the 

 filament, the original septa or inner walls are broken 

 through, and their remains may often be seen on the 

 inner surface of the sheath, appearing as little teeth. 



Oscillatoria nigra (PL VI. fig. 2) is another very 

 similar species, forming blackish-green masses, and 

 is common in ditches. It has longer filaments than 

 the last, with narrowed and slightly curved ends ; and 

 the endochrome is distinctly granular. 



In two other genera of this family, Vib'rio and 

 Spirulma, the filaments are spiral. Vib'rio spiril'lum 

 is excessively minute, colourless, and found in decom- 

 posing vegetable mixtures. The short filaments move 

 rapidly through the water, with a corkscrew-like 

 motion. In Spirulina oscillarioides (PL VI. fig. 12), 

 which is more rarely found in clear pond- waters among 

 Conferva, the filaments are greenish, and form a 

 beautiful simple spiral, resembling that of a very 

 slender spiral vessel. 



Lyngbya murdlis (PL V. fig. 2) is very common on 

 damp walls, gravel walks, &c. It forms a bright 



