110 TAGE SKOGSBERG 



ThcdifferenimfthiHh try ill maiiv Wiivs t(i decrease this consumption of energy by increasing their power of 

 "I ''^'-'■'■""'"^ ""• passive buoyancy. 



pmver of ixissivc ' ' » ^ . 



,n„„i„nn,. Accordiug to ^^ . Ost\\'ald's works (1902, 1903a and b) 



• r 1 -^ Ubergewicht 



,,binkgeschwuidigkeit = \ -_ 



Forinwiderstand x Viskositat des Wassers". 



Of these tliree factors the overweight and the form-resistance have been denoted 

 as the biological ones, because they are dependant on the organism. An increase in 

 the power of buoyancy is thus produced (if the viscosity = the inner friction in the 

 water is constant) by the overweight being decreased and the form resistance in- 

 creased. 



According to A. Steuer, 1910, p. 190, a decrease of the organism's overweight in the 

 plankton world has been produced in the following ways: 



1) by the secretion of mucus and the development of jelly substance, by the development 

 of strongly aqiieoiis tissues, 



2) by the formation of vacuoles, 



3) by the accumulation of specifically light metabolic products, e. g. gas, fat and oil. 

 An increase of the friction resistance in these organisms has been produced, according 



to the same writer (loc. cit.): 



1) by an increase of the whole (relative) surface of the organism (,,Trommeltypus" 

 according to Schroter), 



2) by the organism obtaining a lamellar form (,,Discoplankton" according to Osten'FELD), 



3) by the extension of the body in one direction (rod-shape), 



4) by the develojjment of ,,regelrechter Schwebeapparate", e. g. the long processes in 

 Chaetoceras, a multitude of spines and bristles in Crustacea, etc., 



.5) by the formation of colonies (,,Froschlaichtvpus"). 



What is strived after in these five cases is obviously an increase of the horizontal 

 cross-section. 

 The ri-iaiion .</ ihe What is the relation of the 2)lanktonic Cladocera to these two factors? 



A tendency to decrease the overweight can be observed in all of them. Tliis 

 decrease has in most cases been brought about by the chitin and the tissues of the 

 body becoming finer and by the development of fat. In exceptional cases (Holopedium) 

 it has been produced by the development of a covering of jelly. In a number of species, 

 e. g. Diaphanosoma, the decrease of specific gravity has proceeded so far that the latter has 

 almost coincided with that of the water; these forms can ,,in scheinbar beliebiger Lage im 

 Wasser .stehen' bleiben". In most of the planktonic Cladocera, however, the specific 

 gravity has been decreased rather inconsiderably. 



By what means are the latter forms kept buoyant? 



According to R. Woltereck (and C. Wesenberg-Lund, 1908), in these forms, 

 as in the genus Chy^dorus, the principal factor in preventing sinking is self-motion 

 (swimming). 



iiifthixh. 



