150 



SCIENCE-GOSSIP. 



[July 1, 1866. 



SIMPLE OBJECTS.-No. XIV. 



WATER-FLEAS. 



TN nearly every ditch or pool, at this period of the 

 -*- year, little vivacious crustaceans may be faintly 

 discerned by the naked eye, sporting at will, and ap- 

 parently enjoying themselves in their freedom. The 

 commonest of these are the water-fleas, and the most 

 frequent of all is Daphnia pulex. Several British 

 species are known, but our observations will partake 

 more of a generic than specific character. The 

 woodcuts will convey an idea of the appearance of 

 these creatures under the microscope, as seen by a 

 one-inch objective. The water-fleas belong to the 

 Mntomostraca, a division of the class Crustacea, to 

 which also the fish parasite (A/yul/is) and the four- 

 horned Cyclops, already figured in this journal, 

 belong. 



The body of Daphnia is composed of two portions ; 

 the smaller forms the head, the larger consists of 

 the thorax and abdomen, contained within a delicate 

 shell. The valves of this shell are marked about 

 their centre with a reticulation of faint lines. They 

 are united along the hinder margins, and open 

 in front, the lower extremity in some species 

 being prolonged into a kind of spur of variable 

 length. 



The head is lengthened out in front to a kind of 

 beak, beneath which are the superior antenna? (dif- 

 fering greatly in the two sexes of the same species). 

 In the female these antenna? are very small. On 

 each side, at the base of the head, the larger antenna? 

 are attached. These consist of a single joint, dividing 

 upwards into two branches, the posterior of which 

 has four joints, the other three. Both branches are 

 furnished with long filaments, which in some species 

 are feathered throughout their length. 



The eye of the water-flea is spherical, and com- 

 posed of about twenty lenses. The mouth is a 

 somewhat complicated organ, and consists of a lip, 

 two mandibles, and a pair of jaws, seated near the 

 junction of the head with the body. The stomach 

 is plainly distinguishable as a long vessel, curved 

 upwards at its lower extremity. The body consists 

 of eight segments, the upper of which only is attached 

 to'the shell, An ovoid vesciclc at the back of the 

 first segment, subject to rapid contractions during 

 the life of the animal, is the heart. All the species 

 of true Daphnia have five pairs of legs ; these are 

 variable from each other, and may be observed in 

 motion through the transparent shell. The eggs 

 having attained a certain period of development in 

 the ovaries of the female, are ejected, and from that 

 time until fully mature, are carried in the space 

 between the back of the body and the shell. 



Water-fleas are only found iu fresh water, gene- 

 rally in ponds or ditches, especially where there is 

 plenty of duckweed, and in horseponds by myriads. 



Swammerdam says that he has seen them in such 

 numbers at Vincennes as actually to give the water 

 of the horsepond the colour of blood. Other ob- 

 servers have noticed their communicating to the 

 water in which they swarmed a ruddy tinge. All 

 these creatures increase and multiply at an astonish- 

 ing rate. Muller observes that one act is sufficient 

 to fecundate the mother for life, and all her female 

 descendants for several successive generations. It 

 is certain that males are rarely found, and only in 

 the autumn, whilst females may be obtained through- 

 out the year.* 



Fig. 149. Daphnia pulex (female). 



This brief account could hardly be considered 

 complete without an enumeration of the British 

 species. According to Dr. Baird there are seven, 

 the names and characters of which are as fol- 

 lows : — 



Tfie common Water-flea (Daphnia pulex) 

 found in almost every pool or ditch of standing 

 water during nine months of the year (fig. 149, 

 female, fig. 150, male). 



The Parrot -beak Water-flea {Daphnia psit- 

 tacea) which has the front part of the head beaked 



* A great amount of interesting and valuable information 

 relative to this genus may be found in Baird's " British 

 Entomostraca," page 62 to loo. 



