CRUSTACEA OF FRESH WATERS 163 



the back downwards. Particles of mud and of animal 

 and vegetable matter are drawn by the currents pro- 

 duced in swimming, into the ventral groove between 

 the pairs of feet, and are passed forwards to the 

 mouth to serve as food. Some species of Conchos- 

 traca are said to swim in the same inverted position ; 

 but Messrs. Spencer and Hall, in the memoir already 

 quoted, state that the Australian Conchostraca swim 

 back uppermost. They attribute the difference in 

 habit between the Conchostraca and Notostraca 

 to the fact that in the former group the valves of 

 the shell can be rapidly closed to protect the soft 

 and vulnerable appendages, while no such protection 

 is possible in the Notostraca. They found on one 

 occasion a specimen of Apus (Notostraca) attacked 

 by three Water-beetles, which were tearing its soft 

 appendages, and they suppose that Apus generally 

 escapes such attacks by swimming upside down. 



The breeding habits of the Branchiopoda are also 

 of interest, from the prevalence in many species of 

 reproduction by unfertilized eggs, or " partheno- 

 genesis." This may go on for many generations, 

 and in Apus, for instance, it is possible to examine 

 thousands of specimens before finding a single male, 

 although, for some unexplained reason, males are 

 sometimes comparatively common. It is probable 

 that males must appear sooner or later, otherwise 

 the series of parthenogenetic generations will come 

 to an end; but it is not certain that this is the 



