40 



Fresh-watee Biology 



PLANCTONt 



It will be noted that a number of forms, especially green algae, are included which 

 are not truly plancton organisms. The frequency with which they are brought in from the 

 smaller ponds and streams by students, makes this seem advisable. 



Except for the general arrangement, no originality is claimed for these keys. We 

 are particularly indebted to the writings of Bessey, Birge, Butschli, Collin, Conn, Eyferth, 

 Hausman, Herrick, Hudson and Gosse, Jennings, Kent, Stokes, Tilden, Van Douwe, West 

 and Wolle. The figures are in part drawn from the objects themselves, a few from photo- 

 graphs and some from pubhshed papers. 



ALGAE 



1 — Cells blue-green; reproduction by sunple division. (See PI. 17.) The blue-greens. — 



(Schizophyceae) Cyanophyce ae 2 



— Cells green, red or brown 20 



2 — Cells unicellular or in clusters and colonies, not truly filamentous; commonly em- 

 bedded in a gelatinous matrix, more rarely freely floating Coccogoneae — 3 



— Cells (except Spiridina, fig. 12) filamentous; branched or unbranched, multiplication 

 by filamentous active hormogones Hormogoneae — 11 



3 — Multiplication by vegetative cell division Chroococcaceae — 4 



— Multiplication by conidia. Epiphytes on algae Chamaesiphonaceae 



4 — Cells solitary without gelatinous matrix. Cells spherical Croococcus; cyUndrical 

 (Synechoccus, Chroothece) ; fusiform, (Dactylococcopsis). 

 — Numerous cells in gelatinous matrix 5 



S^Colonies and gelatine without definite form 6 



— Cells in definite arrangement 7 



6 — Cells in several gelatinous capsules. Cells spherical enclosed in shapeless masses of 

 gelatine, Gloeocapsa; cells elongate or eUiptical, Gloeotheca, Zachariasia. 

 — Cells scattered within the gelatinous matrix. Cell spherical (fig. 13), Aphanocapsa; 

 cell elongate Aphanothece 



7 — Colonies free swimming 8 



— Colonies attached Oncobrysa 



8 — Ceil division in 3 planes, colonies therefore in clumps. Cells spherical, colonies more 

 or less spherical, or formed like a bunch of grapes (figs. 8, 9). Microcystis; colonies 

 spherical when young, torn and net like when older, Clathrocystis; cells wedge 



shaped, colonies spherical Gomphosphaera 



— Cell division 2 plane, resulting colonies therefore only one cell deep 9 



9— Colonies a hollow sphere (fig. 10) Coelosphaerium 



— Colonies sheetlike or disclike 10 



10— Cell flattened, disclike (fig. 2) Tetrapedia 



— Cells not flat. Cells spherical (fig. 6), Merismopedia ; cells cylindrical arranged 

 palisade like Holopedium 



(Continued on page 42) 



PLATE 16. SINGLE REPRESENTATIVES OF GROUPS OF LESSER INVERTEBRATES 



1. A hydrachnid, or water mite XIO. 



2. A water spider XI. 



3. A gasterotrich, Chaetonotus XIO. 



4. A coelenterate, Hydra, XIO. 



5. A tardigrade, Macrobiotus X20. 



6. A bryozoan, Plumatella X3 and X30. 



7. A bristle-worm, Nais X20. 



8. A sewage worm, Tubifex X20. 



9. A leech, Clepsine X3. 



10. A flat-worm, Planaria X5. 



11. A colonial rotifer, Conochilus XIO. 



12. A nematode worm XIO. 



13. A fresh-water sponge Xl4- 



14. Gommules and spicules from the same. 



For an introduction to the leeches see Nachtrieb's Leeches of Minnesota. Concerning the other groups see 

 the larger text books of zoology. 



t Reprinted from the Genera of Plancton Organisms of the Cayuga Lake Basin, by O. A. Johannsen and 

 J. T. Lloyd, with additions in the Protozoa and some alterations of the keys by Dr. Johannsen. The plate on 

 Protozoa (21) was arranged by Miss Elsie Broughton. 



