612 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
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7. Monotonic Crustacea-Plankton.—As the crustaceans surpass all 
other classes of the animals of the plankton in quantitative develop: 
ment, so they form monotonic plankton far more often than all other 
classes. Most commonly this simple crustacean-plankton is composed of * 
copepods, not infrequently entirely of a single species (6, pp. 758, 848), 
Next to this I have more frequently found monotonic ostracoda-plank- 
ton; next schizopoda-plankton. Sometimes also there are in these two 
orders only numberless individuals of a single species, sometimes of 
many different species, which compose the monotonic plankton, often 
almost exclusively, and at other times mixed with additions of other 
Crustacea, Sagitta, Salpa, ete. The other above-mentioned orders of 
crustaceans, which also take a considerable part in the constitution of 
the plankton, the decapods, amphipods, and phyllopods, I have never 
found in such quantities that they formed more than half of the mass 
of tow-stuff. On the contrary, such quantities of crustacean-larve of 
one species (e. g., of Lepas and other cirripeds) occasionally appear that 
they predominantly determine the character of the plankton. 
8. Monotonic Tunicata-Plankton.—Next to the monotonic forms of 
plankton, which are composed of Crustacea and Cnidaria, that of the 
Tunicata is most numerous. Quite preponderant in quantity are the 
Thalidie or Salpacee (Salpa and Salpella), and among these, especially 
the smaller species (Salpa democratica-mucronata, S. runcinata-fust- 
formis, and related species). I have often taken such monotonic salpa- 
plankton in the Mediterranean, in the Atlantic and Indian oceans, and 
have received the same also through Capt. Rabbe from different parts 
of the Pacific Ocean. Masses of Doliolwm and of Copelata (Appendicu- 
laria, Vewxillaria, ete.) are also commonly mixed with this in greater or 
less quantities. Still these planktonic tunicates, on account of their 
small size, recede before the Salpe. I know of no instance where they 
have by themselves formed a monotonic plankton. But this is the 
case with the nyetipelagic pyrosoma. The Challenger and the Vettor 
Pisani in the tropics, on dark nights, met with quantities of monotonie 
pyrosoma-plankton in the middle of the Atlantic and Pacific. By day 
not a single one of these “cones of fire” was to be seen, and as soon 
as the moon arose they went into the depths (8, pp. 32, 34). 
9. Monotonic Fish-Plankton.—If, with Hensen, we limit the term 
plankton to the halobios floating passively in the sea, we can desig- 
nate as “monotonic fish-plankton” only the schools of very young and 
small fishes, which often appear abundantly in the currents, occasion- 
ally so compact that very few other pelagic animals can find room 
between them. If one wishes to extend the term still farther, and wipe 
out the sharp distinction between plankton and nekton, all those sea 
fishes (oceanice as well as neritic) which appear in schools, and which 
play so significant an @cological role in the cycle of matter in the sea 
(e. g., Scopelide, Clupeide, Leptocephalide, Scomberoide) will in general 
belong here (12, p. 51). 
