32 



Myxophyceae 



are not infrequent. In both Lyngbya and Anabwna 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 Ccelosphserium, Gompho- 

 sph&ria, 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 Myxophycefe from the plankton. A C, Anabtena Tanganyika G. S. 

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

 All x 520. 



a few species of Myxophyce*, 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). 



