37 6 LAKES AND STREAMS 



tain Apus cancriformis, which has an oval shield, emarginate on the back, 

 and bearing three eyes in front ; the hind part of the body branching into two 

 lono-, many-jointed bristles. In spring this species lives in the mud, to vanish when 

 the water dries up ; but, since the eggs possess self-developing powers, the species 

 often reappears after the lapse of several years. These minute creatures swim on 

 their backs; and not till after the skin has been cast several times are their 

 numerous feet developed. From the last, which has no tin-leaves between the 

 bristles on the tail, the species known as A. p> r oductus is distinguished by a leaf 

 between most of the tail-bristles. The first-named species reappeared recently 

 in England, after having been unknown for many years. 





*m&Jmm 



APUS CAXriUFORMIS 



b ' -shrim Another type, Branchipus stagnalis, which also swims on its 



back, has a narrow, compressed, many-jointed body, with four thread- 

 like feelers, two large eyes, flat-jointed, palmated feet, and a tail branching into 

 two small leaves. This species dwells in fresh water only, but another, the brine- 

 shrimp (Artemia salina), which often causes the water in salt-works to appear red, 

 from its great numbers, is found in pools in the vicinity of dykes that keep out 

 the encroachment of the sea. This tiny crustacean has attracted attention from 

 the fact that in the Crimea, by a gradual increase of salt in the water of the saline 

 lakes, it may become modified into the form known as A. milhauseni, which is an 

 inhabitant of these lakes ; while by diminishing the percentage of salt, it may 

 again be transformed into A. salina. 



