121) TAGE SKOGSHERO 



downward and backward; in this way a progressive movement in a straight line is produced. 

 Alterations in the direction of tht> movement are brought about chiefly by modification in the 

 force of the strokes of one or more of these pairs of limbs. 



If we combine these facts with those put forward on pp. 117 and 118 above, we obtain 

 the following results: 



I. In all planktonic s t r a c o d s whose natatory limbs give the body a j)rogressive 

 movement in a straight line the shell has no processes that effect the direction of the 

 motion. Such forms can Ix' divided into two categories: 



1) Those whose shells are quite without any large processes (Cy p r i d i n i d s, most 

 of the Halocyprids* and all C y p r i d s). 



2) Those whose sheUs have processes. These processes, however, either point in the 

 direction of the movement (the longitudinal axis of the body) or are developed as lateral, 

 symmetrically situated, wing-like formations (a number of Halocyprids). 



II. In the only planktonic Ostracod f Thaumatocypris echinata) whose limbs do 

 not give the body a progressive motion in a straight line the shell has processes that effect 

 the direction of the motion by their position. 



With regard to the processes that are found in the representatives of category I: 

 2 it seems to be beyond doubt that they function as buoyancy organs, as they all contribute 

 more or less to increase the resistance of projection. But this does not seem to be the only 

 funtion. perhaps it is not even the most important one. That this is the case seems to be 

 shown partly by the fact that many of them do not have their maximum extension in the 

 horizontal plane and partly because the forms in which they are developed are very strong 

 swimmers. It is certain that they also function as stability organs. The way in which the 

 posterior part of the shell and the rostrum point in Conchoecia daphnoides — see fig. XVI 

 above — resembles as a matter of fact very much the arrangement of the metal plates that 

 we see on submarines, plates that do not increase the buoyancy power in these vessels, but 

 are designed to increase the stability of the motion (besides influencing its direction). A study 

 of the shape of the shell in this species will show that it very closely approaches the ideal of 

 a swift and stable swimming organism. 



It thus remains to analyse the function of the spines on the shell of Thaumatocypris 

 echinata. In this species, as is seen above, the mechanical arrangements for swimming resemble 

 rather closely those of the genus Bosmina. The body is pressed forward and upward by 

 the natatory strokes of the first and second antennae and for the same reasons as in the 

 last-mentioned species a continual backward rolling movement would be produced if there 

 were no special organs to prevent this. As in Thaumatocypris the natatory limbs are not, 



times in the neighbourhood of the bottom, sometimes higher up, sometimes right up to the surface of the water, 

 about two decimetres from the bottom. In other words it acted in the aquarium in about the same way as most 

 C y p r i d i n i d s. A closer investigation showed that in swimming the first antennae struck upward and backward 

 and somewhat outward, while the exopodite and endopodite of the second antenna and maxilla struck downward and 

 backward and somewhat outward. Alterations in the direction of the motion were produced chiefly by modification 

 of the force of the strokes in one or more of these limbs. 

 • To these belong the Polycopids tog. 



