Daphnia

Daphnia, a genus of Crustacea belonging to the order Cladocera, and including one of the commonest and best known of the "water fleas"; this is Daphnia pulex, which occurs in nearly all ponds and ditches. As it is so common it serves as a good type of this group. The animal has a distinct head prolonged forward into a prominent beak; just beneath this are two pairs of antennas; those of the posterior pair are large, branched, and jointed, and serves as the swimming organ; the anterior pair is rudimentary. From the form of these in Daphnia pulex this species is known as the "branched-horned water flea." The rest of the animal is enclosed in a bivalve shell: the antennae are protruded through an aperture between the valves at the anterior end. There are five pairs of thoracic limbs; the four posterior pair are respiratory in function; they are in constant motion and give the animal a somewhat quivering appearance. The mouth is armed with three pairs of appendages, one pair forms the labrum, and behind this are the mandibles and jaws. On the head there is a single eye, very large in comparison to the size of the animal; it has about twenty lenses; it is a very familiar microscopic object. The reproduction of the Daphnias contains one point of great interest. The males are very scarce in proportion to the number of the females, but one fecundation of these lasts for life. The females produce two types of eggs; the "summer eggs" are shed into a cavity between the back of the animal and its carapace, and remain there till hatched; successive broods of these are formed, the individuals of which are all females, and these can produce other generations of females without any sexual intercourse. At the end of summer this method of reproduction comes to an end, the female is fertilised and forms only a pair of eggs which are retained in an "ephippium," a saddle-shaped cavity in the upper part of the carapace; on the death of the parent Daphnia the ephippium is cast off and floats about, and the eggs are not hatched till spring. The asexual reproduction by the formation of the summer eggs is one of the typical cases of parthenogenesis (q.v.). Baird recognises seven English species of Daphnia, of which D. pulex is by far the most common.