94 THE AMERICAN MONTHLY [Mar 



of the water, I found them covered with flesh -colored worms 

 or larva about 1-16 inch long, and soon small sand fleas 

 appeared. These soon grew large enough to attack the 

 larva which they soon devoured and then commenced upon 

 the diatoms and algse. In a few weeks nothing remained 

 except the crustaceans and an increased sediment, a careful 

 examination of which showed not a trace of diatoms. Not 

 even a fragment of the shells of the large species forming 

 the original sediment remained. 



As many of the cultures had altogether too many dia- 

 toms for healthy growth, I drew up portions of the film with 

 a dropping tube and made other cultures that thus consisted 

 chiefly of salt water and diatoms with very little sediment. 

 Some of the original cultures that had developed too many 

 animals and had lost their diatoms were boiled, and after 

 serating sufficiently a new stock of diatoms added. From 

 others I carefully poured off the salt water and used it as a 

 culture without addition, boiling the sediment in a large 

 quantity of fresh water which was reserved as a nutriment 

 for the cultures. The boiled sediment was cleaned and slides 

 made from the diatoms obtained from it, to be used for com- 

 parison with those obtained from the cultures later. 



In this way, I soon had a stock of over 50 cultures, large 

 and small, under observation. The most ubiquitous and 

 also troublesome of the animals were the anguilla, or "eels." 

 They appeared everywhere in spite of every care, but the 

 most obviously destructive to the Pleurosigma were the 

 ciliata. An oyster-shaped species, large enough to contain 

 15 or 20 Pleurosigma americanum at once, devoted them- 

 selves to the destruction of this kind entirely, while the lit- 

 tle ones devoured the corresponding Pleurosigma, but the 

 amebse took both diatoms and spores. The proper habitat 

 of Pleurosigma americanum is the estuary or tidal inlet 

 at the mouth of fresh water creeks where the tide ebbs 

 and flows, and I supposed from this that they would not 

 be very sensitive to changes, but to my surprise I found 



