52 



aquatic ILitt 



ted hydrogen. On the other hand, an 

 abundance of green algae, like Scenedes- 

 mus, Closterium and so on, is a guar- 

 antee for good health of the fishes, secur- 

 ing aeration of the aquarium through- 

 out. 



The aquarist, who is at the same time 

 a microscopist, may have the opportunity 

 of seeing- unrolled before his view a 



growth on the glass ; the whole com- 

 munity of minute beings plays its part in 

 relation to the health of the fishes ; the 

 larger plants, discharging oxygen in the 

 process of photosynthesis, create the con- 

 ditions necessary for the existence of the 

 microscopically small inhabitants of the 

 aquarium, which again will dispose in 

 smaller or greater degree of the refuse 



Distal Ends of Modified Anal Fins of Male Viviparous Poeciliids 



The structure of the so-called intromittant organ in these fishes is a stable diagnostic char- 

 acter and has played an important part in recent studies of the group. 1. Mollienisia sphe- 

 nops. 2. Gambusia holbrooki. '6. Cnesterodon decemmaculatus. 4. Mollienisia formosa. 5. 

 Phalloceros caudomaculatus. Photomicrographs by Dr. E. Bade. 



regular world-history in miniature, whole 

 nations of infinitesimal animalcules com- 

 ing and going, one preying on the other, 

 one holding the other in check or bal- 

 ance, and each having its own particular 

 task. The infusoria prey on the bac- 

 teria, they themselves being devoured by 

 other "carnivorous" infusoria ; the ro- 

 tifers bunt both kinds of infusoria; the 

 crustaceans live on algae as well as roti- 

 fers; the -nails check the excessive 



matter of the larger inhabitants. Most 

 assuredly a fascinating cycle of life. And 

 this constant automatic purification pro- 

 cess not only reveals to the careful ob- 

 server natural law after law in the small 

 cosmos of his aquarium, but also will 

 permit him to draw more general con- 

 clusions as to the greater laws of the 

 universe, and even as to how human 

 life and his own existence are dependent 

 on natural laws, teaching him that these 



