PLANKTONIC STUDIES. 599 
other hand surpass the former, not only by a great diversity of genera 
and species, but particularly from their enormous development in all 
parts of the ocean. Clio and Limacina are known to occur in the 
Arctic and Antarctic ocean in schools so vast as to form the chief food 
supply of the whales; the swarms of Creseis, Hyalea, and others which 
appear in the seas of the warmer and temperate zones, are also So con- 
siderable that these fluttering “sea buttertlies (farfalle di mare)” often 
play a very important part in the “cycle of matter in the sea” (Stoffwechsel 
des Meeres”). The irregularity of the distribution and phenomena 
is also shown by the fact that Hensen, during his plankton expedition 
through the North Sea (July and August, 1887), completely missed the 
pteropods (9, p. 59; 10, p. 116). On the other hand, when in August, 
1879, I fished at Scoury, on the northwest coast of Scotland, we found 
such immense quantities of Limacina (during the forenoon in still 
weather) that these pteropods certainly formed more than nine-tenths 
of the entire plankton, and with a bucket we could scoop up many 
thousands. The mass of the swarm had the same density for a depth of 
two fathoms and for more than a square kilometer in horizontal extent. 
Cephalopods.—Although entirely swimming animals, these highly 
developed mollusks for the most part do not fall under the term plankton, 
if with Hensen we limit this to those “animals floating involuntarily 
in the sea” (9,p.1). They must then be included in the “nekton;” 
but naturally it depends in some cases entirely on the strength of the 
current whether the small cephalopods should be included in the 
former or in the latter. In any case this highest developed class of 
mollusks is of very great importance in the physiology of the plankton, 
the question of the ‘cycle of matter in the sea.” On the one hand 
they daily consume vast masses of pteropods, crustacea, sagitta, medu- 
sz, and other planktonic animals; on the other, they furnish the most 
important food for fishes and cetaceans. From recent investigations 
itis found that the cephalopods are partly pelagic, partly zonary or 
bathybic (Spirula, Nautilus, etc.). Characteristic small, transparent 
Decolene (Loligopside) are known as partly pelagic, partly bathybie 
species (15, p. 36). The same is true also of some Octolenw (Philonexide). 
Young forms of cephalopods are captured swimming in the plankton at 
the surface as well as in the depths. 
G.—ECHINODERMS OF THE PLANKTON. 
The rayed animals in their significance in the plankton, as also in 
many other morphological and physiological relations, show highly 
peculiar and varied conditions. Although all echinoderms are without 
exception purely marine animals, and no single form of this great 
eroup inhabits fresh water, still not a single species has completely 
adopted the planktonie life. Not a single echinoderm in its full-grown 
and sexually mature condition can be called pelagic. The few forms 
which temporarily swim about (Comatulidw) belong only to the neritic 
