102 Aquatic Organisms 



The cells sometimes form irregular masses, with more or 

 less gelatinous investiture. Often they form simple 

 threads or filaments, or flat rafts, or hollow spheres. 

 Algal filaments are sometimes simple, sometimes 

 branched; sometimes they are cylindric, sometimes 

 tapering ; sometimes they are attached and grow at the 

 free end only ; sometimes they grow throughout ; some- 

 times they are free, sometimes wholly enveloped in 

 transparent gelatinous envelopes. And the form of the 

 ends, the sculpturing and ornamentation of the walls and 

 the distribution of chlorophyll and other pigments are 

 various beyond all enumerating, and often beautiful 

 beyond description. We shall attempt no more, there- 

 fore, in these pages than a very brief account of a few 

 of the commoner forms, such as the general student of 

 fresh water life is sure to encounter; these we will 

 call by their common names, in so far as such names 

 are available. 



The flagellates — We will begin with this group of 

 synthetic forms, most of which are of microscopic size 

 and many of which are exceedingly minute. That 

 some of them are considered to be animals (Mastigo- 

 phora) need not deter us from considering them all 

 together, suiting our method to our convenience. The 

 group overspreads the undetermined borderland be- 

 tween plant and animal kingdoms. Certain of its 

 members {Euglena) appear at times to live the life of a 

 green plant, feeding on mineral solutions and getting 

 energy from the sunlight; at other times, to feed on 

 organic substances and solids like animals. The more 

 common forms live as do the algae. All the members of 

 the group are characterized by the possession of one or 

 more living protoplasmic swimming appendages, called 

 flagella, whence the group name. Each fiagellum is 

 long, slender and transparent, and often difficult of 



