The Anostraca and Notostraca usually swim with the back downwards. Particles of mud and of animal and vegetable matter are drawn by the currents produced 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 Conchostraca 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 "parthenogenesis." 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 case, and there are some species of Conchostraca of which the males have never been seen. [Illustration: FIG. 55--THE BRINE SHRIMP (_Artemia salina_). (After Sars.) A, Female, under-side, × 6; B, head of male, upper side, further enlarged, showing the large clasping antennæ. The larval stages of this species are shown in Fig. 33, p. 81]