' studies on marine Ostracods 111 



The first-mentioned writer has in the case of these forms accordingly written the above- 

 quoted buoyancy formida established by W. Ostwald, in the following way: 



„. , , . -. , . trbergewicht x Abwartsbewegung und -steuerung 



„Smkgeschwmdigkeit = — -_ — 



Reibung x Aufwartsbewegung und -steuerung". 



The factor ,, Reibung" in this formula, which includes both the viscosity of the water 

 and the friction between the sinking body and the water, i. e. the form-resistance, appears, 

 according to this author, in these forms (as in the actively swimming forms of Crustacea in 

 general) not to have ,,eine besonders groBe Bedeutung", p. 480. 



The shape of the body in the pelagian Cladocera varies, however, to a very great extent; 

 many of them are characterized, for instance, by more or less excessively developed processes 

 of different kinds. 



How are these processes to be explained? How are the more oi 



The most widelv spread view seems to be that these processes are to be explained **° *'^^"^"'" P"""' 



^ r r cesses on the shell oj 



as buoyancy organs. the Cladocera to bt 



A view that differs somewhat from this is put forward by C. Wesenbbrg-LUND; explained? 

 in a work of 1908 this eminent author writes as follows (p. 12): „I am inclined to Uii'«>!if"icy organs. 

 believe on the whole, that many of these buoyancy-organs which have hitherto been 

 considered of importance only in as far as they increase the cross-section resistance 

 and surface-area, play a by no means small role in shifting the centre of gravity of the 

 body", i. e. a number of these processes function as balance - organs as well as buoyancy "-' ^«'°"<=^ organs. 

 organs. In support of this view observations made on the genus Bythotrephes are given 

 in the work mentioned; thus we read (p. 12): ,,As mentioned above I have never seen 

 Bythotrephes floating; it hops about always in the aquaria with innumerable, small and 

 short jumping movements; it gives one the impression of being an excellent swimmer but 

 not a floating organism. I have never seen it use its long, posterior legs as outriggers; it drags 

 the long spine behind it during swimming and we might think on a cursory glance that this 

 was rather a hindrance. The direction of movement is as a ride distinctly horizontal. If we 

 now remove the spine, which can easily be done with a good pair of scissors, we change the 

 Bythotrephes to a dancing figure, waltzing round and round in spirals or closed circles; they 

 finally end at the bottom, from which they never again rise. The spine has thus actually 

 been a balancing organ, which has played the role of moving the centre of gravity, so 

 that a horizontal movement could be possible; further it is a buoyancy organ, which accor- 

 ding to its point of insertion and the position it gives the body in the water augments 

 the cross-section resistance." 



The same view (which was afterwards adopted by F. E. RtHE, 1912, among other writers) 

 had already been previously (1896) expressed by C. CHUN with regard to a nmuber of other 

 pelagian Crustacea. After an expression of this opinion we read in the work mentioned (p. 103): 

 „Sind die betrefienden Formen mit kraftigen RuderfiiBen versehen, so liegen die Balancirstangen 

 horizontal in der Mediane vmd bedingen bei dem Durchschneiden des Wassers eine geradlinige 

 Fortbewegung und einen geringen Widerstand," 



