598 KEPORT OF COMMISSIONER OF FISH AND FISHERIES. 



(Trochosphufd, Ichthydinn, Rotifera) ; (li) the Strongylarkv {Xematoda^ 

 Acanthocephala, Chwtnynathu); (3) tlie Jihynchoca'Ia {Nemcrtina, En- 

 teropneusta), and (4) the Prosopi/ghv {Bryozoa, Brachiopoda, Phoronea', 

 SipuHCulcd'). Tlie larvjc of many of tliese wo-ims have acquired the 

 pelagic mode of life, but most of them are too small and too scattered 

 iu the plankton to be of any considerable importance in its composition. 

 GhicttHjnatha. — lu its mature condition only a single class of hel- 

 minths plays an independent and indeed an imi)ortant role in the plank- 

 ton — the small and peculiar class of arrow- worms or Cluvtognatha 

 {Sagitta, Spadella, etc.). These, together with the copepods, salpae, 

 pteropods, and radiolarians belong- to the most substantial, most gen- 

 erally distributed, and usually unfailing constituents of the plankton. 

 Hensen (9, p. 50) has made some calculations of the immense numbers 

 in which they appear. He reckons them in the "perennial plankton," 

 yet does not find "everywhere the regularity which one might expect." 

 He is astonished at the "highly remarkable variations" in their num- 

 bers, and finds this very unequal distribution very puzzling- (0, p. GO). 

 Chun has lately shown that the troops of Sagitta not only populate the 

 surface of the sea, but also " in common with the Eadiolciria, Tomop 

 teridcc, IHphyidw^ Grnfitaeca, coustitiite the most numerous and most 

 constant inhabitants of the greater depths. In countless multitudes 

 they are taken in the open as well as in the closible net, from 100 

 meters down to 1,300 meters" (15, p. 17). It seems that iSagitta, as a 

 whole purely oceanic, is represented by pelagic as well as zouary and 

 bathybic species. 



F. — MOLLUSKS OF THE PLANKTON. 



The race of mollusks play a very important role in the plankton. 

 Although the great majority of the genera and si)ecies belong to the 

 benthos, yet there are a few families which have become adapted to the 

 pelagic mode of life, of great importance on account of the great 

 swarms in which they often appear. The three chief classes which we 

 distinguish in this race (30, p. 546) live very differently. The Acephala., 

 entirely benthonic, can take part only as swarming larva3 in the com- 

 position of the i»lankton; so also the swimming larvte of many mero- 

 planktonic GaHtropoda. Of these latter only a very few genera have 

 adopted completely the pelagic mode of life, like lanthina among the 

 prosobranchs, GIuucvh and PliyUirrha' among the opisthobranchs. 



PtcropodH and Heferopods. — These two groups of snails are holoplank- 

 tonic, chietly nyc^tipelagic animals, which come to the surface of the sea, 

 preferably during the night, in vast numbers (14, pp. 121-125). Chun 

 has lately discovered that many of them are found at considerable 

 depths (15, p. 30). Some kinds of pteropods {c. g., tSpirialis) seem to 

 belong to the zonary and bathybic fiiuna. The heteropods are on the 

 whole of less importance. They occur in great swarms less frequently 

 and only in certain parts of the warmer seas. The pteropods on the 



