634 THE WATER-FLEA. 



The figure of the young Brine-shrimp, which is greatly magnified, is introduced for 

 the purpose of showing the great dissimilarity of form in the young and the adult. 

 Indeed, through the whole of the Entomostraca, the young bear a great general re- 

 semblance to each other, but are quite unlike their parents. 



THE two larger figures represent the FAIRY-SHRIMP, appropriately named, as a more 

 fairy-like creature can hardly be conceived. It is to be found in several parts of England, 

 and there is a pond on Blackheath where it used to be tolerably common. In spite of its 

 comparatively large size, measuring more than an inch in length, it may easily escape 

 observation, as its body is of glassy transparency, and scarcely visible in the water ex- 

 cept by the red and blue tints of its tail, branchiae, and feet. It always inhabits stag- 

 nant water, and may even be found in the half putrid mass of mud and water that lies 

 at the bottom of casting nets. 



In his valuable work on the British Entomostraca Dr. Baird gives the following 

 account of this beautiful creature : " They swim upon their back, and in fine, warm 

 weather, when the sun is not too strong, they may be seen balancing themselves as it 

 were, near the surface, by means of their branchial feet, which are in constant motion. 

 On the least disturbance, however, they strike the water rapidly with their tail from right 

 to left, and dart away like a fish, and hasten to conceal themselves by diving into the 

 soft mud, or amongst the weeds at the bottom of the pool. 



It is certainly the most beautiful and elegant of all the Entomostraca. The male is 

 especially beautiful. The uninterrupted undulatory waving motion of its graceful 

 branchial feet, slightly tinged as they are with a light reddish hue ; the brilliant mix- 

 ture of transparent bluish green and bright red of its prehensile antennae, and its bright 

 red tail, with the beautiful plumose setae springing from it, render it exceedingly attrac- 

 tive to the view. 



The undulatory motion of its branchial feet serves another purpose in addition to that 

 of keeping the animal suspended in the water. The thorax or body of the animal has 

 been described, when floating on its back, as like the cavity of a little boat, the feet 

 representing the oars. When these are in motion, they cause the water contained in 

 this cavity to be compressed, and to mount up as along a canal, carrying in the current 

 the particles destined for its food towards the mouth. It seems to be constantly, when 

 in this position, employed in swallowing and digesting its food, its masticatory organs 

 being in perpetual motion." 



In the illustration, the upper figure represents the male, and the lower the female. 



WE now come to some of the minute species of these curious animals, and, in 

 consequence, all the figures are much magnified. The creatures belonging to this order 

 are termed the Cladocera or Branch-horns, because their antennae are forked and 

 branched. With the exception of the head, the body is wholly enclosed within two shells 

 like the half shells of a walnut. There is only one eye, of very large comparative size ; 

 there are two pairs of antennae, and the lower pair are used in swimming. In the first 

 family, the Daphniadae, the lower antennae are very large and upper very small. There 

 are five or six pairs of feet enclosed within the shell, and the intestine is straight. 



The common WATER-FLEA is a good example of the typical genus. Eight or nine 

 species of this genus are now known, and the habits are much the same in all. They 

 are to be found in fresh water, whether still or running, but seeming to prefer the 

 former. Even the horse-ponds, which are so trodden about by cattle that they seem to 

 consist of equal parts of bad pea-soup and duckweed, are favored residences of the 

 Water-fleas, and several species may sometimes be found in one little pond. 



The common Water-flea has five pairs of feet, the lower antennae branched and 

 branchleted, the lower pair being very large and powerful. The head is lengthened 

 downwards into a prominent beak. These little creatures exist in vast numbers, and 

 sometimes assemble in such heavy masses that they quite change the color of the water. 

 Dr. Baird has remarked that they will sometimes assemble so as to form a belt of a foot 

 or so in breadth, and ten or twelve yards in length, and that the whole belt will pass 



