AQUATIC CRUSTACEANS 29 



shrinks when not required for this purpose. The headpiece 

 of the carapace often forms a prominent curved beak, giving 

 the animal a most curious appearance. 



The food of these little crustaceans is various; usually it 

 consists of decomposing organic matter. Apus, a species of 

 Phyllopoda now thought to be extinct, has been seen to 

 attack tadpoles, whilst Daphnia, being less of a gourmand, is 

 satisfied with a diet of mud. 



The limbs, of which there are never more than four pairs in 

 the Cladocera, are in certain species most beautiful objects, 

 those of the female Simocephalus being adorned with hairs, 

 resembling the most perfect feathers, of microscopic size. When 

 the creature is swimming the carapace is usually open, but 

 should the water-flea wish to rest the " shell " acts as an 

 anchor by gripping the weed. 



Leptodora hyalina and Daphina Pulex are two of the largest 

 British species, and are therefore of particular interest as they 

 afford examples for study. 



Daphnia Pulex can be readily recognised by its size and pro- 

 minent beak. Owing to the transparency of both carapace and 

 body, one is able to watch natural functions with perfect ease. 

 The heart, a peculiar cleft organ, will be seen to be actively 

 "beating." The digestive tract can be examined from end to 

 end (not a very great distance) and in certain species the young, 

 which remain inside the maternal carapace until mature, can be 

 seen to have inside their little carapaces a yet younger gene- 

 ration. 



Fortunately for the water-flea and its admirers, asexual 

 reproduction is constant. The male is a very rare creature, 

 and only appears in meagre numbers at particular times. In 

 fact hejmay be labelled as a rarity. He resembles the female, 

 but is much smaller. Water-fleas produce two kinds of eggs, 

 winter and summer eggs. The former, which are fertilised 

 eggs, are produced on the approach of cold weather or other 

 critical periods. 



The carapace of the female becomes thickened, and the eggs, 

 either one or two, are enclosed therein. The creature then 

 moults, and the discarded carapace containing the eggs rests 

 on the mud until conditions are more favourable. These winter 

 eggs are most hardy in structure, and can resist freezing and 

 drying with impunity. 



The carapace and eggs, which are very light, may be blown 

 by the wind from a dried-up pond, or carried by birds or other 

 animals to fresh water. Hence water-fleas often make a sudden 

 appearance in water where they have not previously been found. 



