598 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
(Trochosphera, Ichthydina, Rotifera) ; (2) the Strongylaric (Nematoda, 
Acanthocephala, Chetognatha); (3) the Rhynchocela (Nemertina, En- 
teropneusta), and (4) the Prosopygie (Bryozoa, Brachiopoda, Phoronea, 
Sipunculee). The larve of many of these worms have acquired the 
pelagic mode of life, but most of them are too small and too scattered 
in the plankton to be of any considerable importance in its composition. 
Chetognatha.—In its mature condition only a single class of hel- 
minths plays an independent and indeed an important role in the plank- 
ton—the small and peculiar class of arrow-worms or Chctognatha 
(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. 59) 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 (9, p. 60). 
Chun has lately shown that the troops of Sagitta not only populate the 
surface of the sea, but also “in common with the Radiolaria, Tomop- 
teride, Diphyide, Crustacea, constitute the most numerous and most 
constant inhabitants ef 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 Sagitta, as a 
whole purely oceanic, is represented by pelagic as well as zonary and 
bathybic species. 
F.—MOLLUSKS OF THE PLANKTON. 
The race of mollusks play a very important réle in the plankton. 
Although the great majority of the genera and species 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 larve in the com- 
position of the plankton; so also the swimming larve of many mero- 
planktonie Gastropoda. Of these latter only a very few genera have 
adopted completely the pelagic mode of life, like Zanthina among the 
prosobranchs, Glaweus and Phyllirrhe among the opisthobranchs. 
Pteropods and Heteropods.—These two groups of snails are holoplank- 
tonic, chiefly nyctipelagic 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. 36). Some kinds of pteropods (e. g., Spirialis) seem to 
belong to the zonary and bathybie fauna. 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 
