612 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



7. Monotoniv Crustacea -Plankton. — As the, crustaceans suriiass all 

 other classes of the animals of the i>lanktoii in (jiiantitative develop- 

 ment, so they form nionotonic X)lankton far more often than all otln^r 

 classes. Most commonly this simple crnstaeean-plankton is coniposcd of 

 copepods, not infrequently entirely of a single species (6, pp. 758, 843). 

 Next to tliis I have more frequently found monotouic ofitracodapIanJc- 

 ton; next schizopoda-iilaxlcton. Sometimes also there are in these two 

 orders only numberless individuals of a single species, sometimes of 

 many different species, which compose the monotonic i)lanlvton, often 

 almost exclusively, and at other times mixed witli additions of other 

 Crustacea, Sagitta, Sal^ja, etc. The other above-mentioned orders of 

 crustaceans, which also take a considerable part in the constitution of 

 the plankton, tlie 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 crustaccau-Iarvcv of 

 one species (e. g., of Lepas and other cirri])eds) occasionally appear that 

 they predominantly determine the character of the plankton. 



8. Monotonic Tunicata-PlanJcton. — Next to the monotonic forms of 

 jjlankton, which are composed of Crustacea and Cnidaria, that of the 

 Tunicataii^ most numerous. Quite preponderant in quanlity are the 

 ThaUdiw or ^alpacew (ISalpa and ^SaI2)eUa), and among these, especially 

 the smaller species {Salpa dcmocratica-mucronata, S. runcinata-fusi- 

 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. Kabbe from different parts 

 of the Pacific Ocean. Masses of DoJiolum and of Copelata {Appcndicu- 

 laria, Vexillaria, etc.) are also commonly mixed with this in greater or 

 less quantities. Still these jplauktonic tunicates, on account of their 

 small size, recede before the Salpa\ I know of no instance where they 

 have by themselves formed a monotonic ])lankton. But this is the 

 case with the nyctipelagic pyrosoma. The Challenger and the Vettor 

 Pisani in the tropics, on dark nights, met Avith ciuantities of monotonic 

 pyrosoma -planMon 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 halohio.s tioating i)assively in the sea, we can desig- 

 nate as "monotonic fish-plankton"' only the schools of very young and 

 small fishes, which often aiipear . abundantly in the currents, occasion- 

 ally so compact that very few other pelagic animals can find room 

 betAveen them. If one wishes to extend the term still tarther, and wipe 

 out the sharp distinction between plankton and nekton, all those sea 

 fishes (oceanic as well as neritic) which appear in schools, and which 

 play so significant an (ecological role in the cycle of matter in the sea 

 {e. //., Scopelidce, Clnpcidw, Leptocephalidfc, Scomheroidm) will in general 

 belong here (12, p. 51). 



