206 77/r Onistfwea of Cambridgeshire 



serve as a typical Cladoceran. With the exception of the 

 head, the creature- is laterally tlattrneil and enclosed in a 

 bivalve shell, while locomotion is effected ly I lie powerful 

 strokes of the large second antennae. M////////W //////-./ is one 

 of the commonest and best known species, and occurs in this 

 district in great abundance. Very common also is >'/'// 

 phalus sima, which was formerly included under the heading 

 Daphnia. Closely allied we have also Daphnia mayna and 

 1). longispina. 



All these forms are active swimmers, but the style of 

 swimming affords a ready means of distinction between the 

 genera Daphnia and Simocephalus. The members of t In- 

 former genus swim in a vertical position, head uppermost, 

 by constant beating of the large biramous second antennae. 

 Simocephalus, on the contrary, swims on its back with some 

 vigour, but never for long without coming to rest. Since it 

 has been demonstrated that the movements of the swimming 

 antennae are in both cases almost identical, we can only 

 believe that the centre of gravity must be quite differently 

 situated in the two genera. 



A further very characteristic attitude of SimocephcUus is 

 that which it assumes on coming to rest, when it hangs on 

 to water- weeds or the like, by means of minute hooks at the 

 end of the second antennae. This position is evidently a 

 favourable one for respiration and the taking in of food, for 

 both processes are effected by a regular beating of the five 

 pairs of legs, to produce a circulation of the water between 

 the valves of the shell. Some idea of the importance of this 

 movement of the legs may be gathered from the fact that 

 they usually perform no less than 300 beats within the minute. 

 It is easy to understand that all light particles of organic 

 matter would be carried by the current of water within reach 

 of the mouth appendages, and that the food of such forms 

 thus consists entirely of small unicellular organisms with both 

 animal and vegetable detritus. 



The process of shedding the skin, which is periodically 



