544 



CRUSTACEA. 



of whicli the Gladocera present the nearest approach to the pre- 

 ceding, having a bivalve carapace, no more than from four to 

 six pairs of legs, two pairs of antennae, of which one is large and 

 branched and adapted for swimming, and a single eye. The 

 commonest form of this is the Daphnia pulex, sometimes called 

 the "arborescent water-flea!' from the branching form of its an- 

 tennae. It is very abundant in many ponds and ditches, coming 

 to the surface in the mornings and evenings and in cloudy 

 weather, but seeking the depths of the water during the heat of 

 the day. It swims by taking short springs ; and feeds on minute 

 particles of vegetable substances, not, however, rejecting animal 

 matter when offered. Some of the peculiar phenomena of its 

 reproduction will be presently described (§ 370). The order 

 Phyllopoda includes those Branchiopoda whose body is divided 

 into a great number of segments, nearly all of which are fur- 

 nished with leaf-like members, or "fin- feet." The two families 

 which this order includes, however, differ considerably in their 

 conformation ; for in that of which the genera Apus and Nehalia 

 are representatives, the body is enclosed in a shell, either shield- 

 like or bivalve, and the feet are generally very numerous ; whilst 

 in that which contains Branchipus and Artemia, the body is en- 

 tirely unprotected, and the number of pairs of feet does not 

 exceed eleven. The Apus cancriformis, which is an animal of 

 comparatively large size, its entire length being about 2J inches, 

 is an inhabitant of stagnant waters ; but although occasionally 

 very abundant in particular pools or ditches, it is not to be met 

 with nearly so commonly as the Entomostraca already noticed. 

 It is recognized by its large oval carapace, which covers the 

 head and body like a shield; by the nearly cylindrical form of 

 its body, which is composed of thirty articulations ; and by the 

 multiplication of its legs, which amount to about sixty pairs. 

 The number of joints in these and in the other appendages is so 

 great, that in a single individual they may be safely estimated at 

 not less than two millions. These organs, however, are for the 

 most part small ; and the instruments chiefly used by the animal 

 for locomotion are the first pair of feet, which are very much 

 elongated (bearing such a resemblance to the principal antennae 

 of other Entomostraca, as to be commonly ranked in the same 

 light), and are distinguished as rami or oars. With these they 

 can swim freely in any position ; but when the "rami" are at 

 rest and the animal floats idly on the water, its fin-feet may be 

 seen to be in incessant motion, causing a sort of whirlpool in 

 the water, and bringing to the mouth the minute animals (chiefly 

 the smaller Entomostraca inhabiting the same localities) that 

 serve them as food. The Branchipus stagnalis has a slender, 

 cylindriform, and very transparent body of nearly an inch in 

 length, furnished with eleven pairs of fin-feet, but is destitute of 

 any protecting envelope; its head is furnished with a pair of 

 very curious prehensile organs (which are really modified an- 



