92 Clark — Holophytic Plankton of Lakes Atitlan and Amatitlan. 



taken only in horizontal hauls in sheltered bays, and may on 

 this account have escaped. 



The organisms mentioned under the holophytic zooplankton 

 are by some authorities counted among plants, and by others 

 among animals. In some cases the only reason for their being 

 included in the vegetable kingdom appears to be the presence 

 of chlorophyl. In structure they are closely allied to animals 

 everywhere recognized as such and the presence of chlorophyl 

 appears to be simply a physiological adaptation, much as the 

 absence of chlorophyl among parasitic and saprophytic plants 

 is. The structure of various members of the Volvocacese is par- 

 ticularly significant in this connection, as the organisms as a 

 whole represent embryonic stages well-known in the animal 

 kingdom and indeed necessarj^ to complete the animal series, 

 while they have no particular significance in the plant series 

 and placed anywhere in that group have always the aspect of 

 foreigners. 



In so far as the question of fish food is concerned, or indeed 

 from any point of view, the presence of a small amount of 

 holozoic plankton is much more satisfactory than a large amount 

 of holophytic plankton. A great excess of phyto-plankton is 

 even a disadvantage; in addition to its frequently accumulating 

 in masses as a scum on the surface or along the shore, and there 

 decaying and becoming an offence both to sight and smell, it 

 may sink to the bottom in masses where its decay induces con- 

 ditions unfavorable to fish life. On the other hand, the presence 

 of holozoic plankton is evidence of at least enough holophytic 

 material to provide food for fishes, and it rarely or never becomes 

 objectionable by its abundance. Most young fishes live on 

 zooplankton, and the phytoplankton is significant mainly as 

 affording pasturage for the zooplankton, and through it for the 

 fish. Even in the case where fishes with herbivorous young 

 are present, the zooplankton offers a more varied food-supply 

 and is of special importance when it comes to the question of 

 introducing other species of fish. 



For example, Amatitlan appears to have much more plankton 

 than Atitlan, but unless the young of the fishes of that lake are 

 herbivorous it does not contain nearly so much fish food, as the 

 plankton is mainly holophytic, relatively few entomostraca being 

 present, while in Atitlan, though there is not nearly so much 



