32 



Myxophyceae 



are nob infrequent. In both Lyngbya and Anabtena certain of the plankton- 

 species have become spirally coiled (fig. 19). In Aphanizomenon the fila- 

 ments are straight and densely aggregated to form floating bundles, a habit 

 which is also characteristic of the marine genus Trichodesmium, a pink 

 species of which gives the colour to the Red Sea. Of the Coccogonese 

 the principal genera found in the plankton are Cwlosphcvrium, Gompho- 

 sphxria, Microcystis, Chroococcus, and Dactylococcopsis, the two former genera 

 sometimes occurring in such quantity as to dominate the entire plankton. 



The phenomena of ' water- bloom ' and ' the breaking of the meres ' are 

 due to the sudden and rather sporadic development of large quantities of 



Fig. 19. Spirally coiled Myxophyceae from the plankton. A C, Anabsena Tanganyika G. S. 

 West; D and E, A. circularis G. S. West; FH, Lyngbya circumcreta G. S. West. 

 All x 520. 



a few species of Myxophycese, more especially certain of those which normally 

 occur in the plankton of lakes and pools. The extraordinary rapidity with 

 which these species multiply, with consequent discolouration of the water, is 

 a fact which is not yet fully understood ; but their disappearance, which 

 is often equally rapid, may be due partly to exhaustion of available food- 

 supplies and partly to the action of toxic substances secreted by themselves. 

 It has been found by Nelson ('03) that water containing ' water-bloom ' has 

 often been fatal to cattle ; and in this connection it is interesting to note 

 that horses have frequently been killed in the Gulf of Manar by feeding 

 upon Lyngbya majuscula (G. S. W., '04). 



