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 a? id temperate zones, are also so con- 

 siderable that these fluttering "sea butterflies {Farfalle di marey often 

 play a very important part in the "cycle of matter in the sea" [StoffwecJisel 

 des Meeres^^). The irregularity of the distribution and phenomena 

 is also shown by the fact that Hensen, during his plankton expedition 

 through the iSTorth Sea (July and August, 1887), completely missed the 

 pteropods (9, p. 50; 10, p. 110). 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 

 w^eatlier) that these pteropods certainly formed more than nine-tenths 

 of the entire lilankton, and with a bucket we could scoop up many 

 thousands. The mass of the swarm had the same density for a deptli of 

 two fathoms and for more than a square kilometer in horizontal extent. 

 Cephidopods. — 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 "uekton;" 

 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 i)hysiology 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- 

 sa?, and other planktonic animals; on the other, they furnish the most 

 important food for fishes and cetaceans. From recent investigations 

 it is found that the cephalopods are j)artly pelagic, jiartly zonary or 

 bathybic {Spirilla, ¥autilus, etc.). Characteristic small, transparent 

 Deeolenie {LoUgopsidtc) are known as partly pelagic, partly bathybic 

 species (15, p. 36). The same is true also of some Octolenw {Philonexidw). 

 Young forms of cephalopods are captured swimming in the plankton at 

 the surface as well as in the depths. 



G. — ECIIIXODEKMS 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 i)urely marine animals, and no single form of this great 

 group inhabits fresh water, still not a single species has completely 

 adopted the planktonic life. Not a single echinoderm in its full-grown 

 and sexually mature condition can be called pelagic. The few forms 

 \y^hich temporarily swim about {Comatalida;} belong only to the neritic 



