170 HOUSE, GARDEN, AND FIELD 



THE WATER-FLEA (DAPHNIA). 



Let us search the pool. I lower a dipper into the water, 

 and bring up a cupful. As soon as the mud has settled, 

 I see a number of whitish specks, which swim with short 

 jerks or run on the bottom ; these are the so-called water- 

 fleas. One is tempted to find fault with the name, for 

 the water-flea has very little in common with the 

 domestic flea, but the name is Swammer dam's, and 

 will not easily be forgotten while Swammerdam continues 

 to be read. 



Every zoological textbook gives a description of the 

 water-flea (Daphnia), and excellent figures abound, so that 

 our account of its structure may be very brief. It re- 

 sembles a very small shrimp, covered by a sort of cope (the 

 carapace), which is made fast to the back of the head, and 

 doubled in two, but not hinged. The carapace is flattened 

 from side to side, and so big that head, body and limbs 

 can be almost completely retracted within it. A pair of 

 large branched antennae are used as sweeps, and it is these 

 which jerk the body through the water ; the other limbs 

 are small, and hardly project from the narrow slit between 

 the edges of the carapace ; the forked tail is turned for- 

 wards beneath the body, and is constantly engaged in 

 a sweeping movement, baling the water out of the enclosed 

 space. Both carapace and body are transparent, allowing 

 us to see the beating of the heart and the streaming of the 

 blood in a live specimen laid on the stage of the microscope. 

 The paired eyes, usually found in crustaceans, coalesce 

 in the water-flea, and form one big compound eye, which 

 is always trembling. Water-fleas show no distinct colour, 

 except where many are collected in a small space, when 

 they are said to give a reddish tint to the water, which is 

 particularly evident in spring. 1 They often swim near 



1 Swammerdam speaks of water being changed to blood, according to 

 popular belief, when examination showed that the colour was due to 

 crowds of Daphnias. This appearance I have never seen myself. 



