studies on marine Ostracods 121 



as in tlie case of the genera Daphnia and Bosmitm, extended at the sides by a shell incisur, and 

 as the body of this species is not, as in the last-mentioned species, suspended during swimming 

 in these excentricaUy situated appendages, its rotatory axis is not, as in Daphnia and Bosmina, 

 situated through the points of attachment of the appendages in question, but presumably just 

 in front of the middle of the shell. The following factors prevent a backward rotation and 

 render possible a progressive moton in a straight Une. The most important factors are 

 the two pairs of spines on the shell. These are situated, as is shown in fig. XVIII above, 

 almost on the continuation of the radii of the almost circidar shell, one pointing almost straight 

 forward (perhaps somewhat downward), the other almost straight downward. As the axis 

 of rotation is presumably situated, as is shown above, just in front of the middle of the shell, 

 this position of the spines as a continuation of the radii of the shell seems to be almost ideal; 

 during rotation the resistance of the water will be directed practically at a right angle to them. 

 The resistance produced by the water against the progressive motion of the body obviously 

 has no effect at all (or at any rate only an exceedingly slight one) on the forward- (or possibly 

 slightly downward-) pointing pair of spines, and, on the other hand, it exerts practically a 

 perpendicular influence on the downward-pointing pair of spines. Thus the lower pair of 

 spines, like the first antennae in the genus Bosmina, operate like a pair of oars that are held 

 out on one side of a moving boat. Just as the oars try to turn the boat in the direction in 

 which they point, so the downward-pointing pair of spines in Thaumatocypris press the anterior 

 part of the body downward, in other words this force, too, opposes the natatory strokes of the 

 antennae, which give an upward turn to the body. Both pairs of spines thus make a pas- 

 sive resistance against a backward rotating movement; the lower pair of spines exercises, 

 in addition, an active downward pressure on the anterior part of the body. One more 

 factxir seems presumably to help to prevent a backward rotating motion; this is the force 

 of gravity, for, as is seen from pi. VI, fig. 2, G. \\. MUller 1906a, the centre of gravity 

 in this species is presumably somewhat in front of the middle of the shell. 



R. Woltereck's theory, which has, of course, by no means been imopposed* thus Summary of the re- 

 seems to me, as I have pointed out above, to obtain very strong support *"'*"/ '"2/ mvesu- 



_ '^ o r ir gallon of the relation 



from the conditions in the Ostracod group. For scarcely anything more »/ the Osiracods lo 

 striking can be imagined than that of all the many planktonic Ostracod species, whose nata- ''"'•' '"'"''f<'">- 

 tory limbs produce a progressive movement in a straight line, not a single one has processes 

 that influence the direction of the motion, while the single form in which the natatory 

 appendages do not produce a movement of this sort has a shell that is equip^jed with 

 these processes. 



All the same it seems to me that R. Woltereck — like many other investigators before /•''«'« remark. 

 him, when they have hit on a productive idea — is inclined to overestimate the importance 

 of his new principle. A careful analysis of the importance of the more or less peculiar 

 shapes of the different plankton organisms will probably show that the different principles, 

 the principle of 1) buoyancy, 2) balance, 3) direction and 4) stability all play a very great part. 



* See, for instance. O. Olofsson: ..Studirn uber die SiiOwasscr f aun a Sp i t z b cPKe n s, eti-." 

 ZooloKiska Bidrag t'r4n Uppsala, Bd. VI. 



Zoolog. hulr.ig, Upp^ahi. Suppl.-P.ii. I. ^° 



