84 DIPS INTO MY AQUARIUM. 



has appendages to the head like branched antlers. Moreover, its 

 body is perceived to be encased in an almost transparent shell, 

 having two valves, while its feet are prolonged into plates very 

 much like the gill-plates of fishes, only, of course, much more 

 diminutive. 



Water-fleas belong to a group of animals which form a sub- 

 division of Crustaceans, so that they are really humble relatives of 

 the crab and lobster. Here is Professor Nicholson's classification 

 of the Crustacea. Provisionally taking Cirripedia or Barnacles 

 as a sub-class of Crustacea, he proceeds to arrange all other 

 animals of this group in two sub-divisions : — i. — Entojnosiraca, 

 including water-fleas — Cyclops, Daphnia, etc. — with the great 

 extinct group of Trilobites, etc. 2. — Malacostraca, which are 

 either (i) sessile-eyed, like the sand-hopper, or (2) stalk-eyed, like 

 the lobster. 



The word entomostraca really means a shelled insect ; and yet 

 water-fleas are not insects, nor have they a shell, properly speaking. 

 But words, after all, are poor things, and perhaps this name is 

 sufficiently accurate to convey even to the non-scientific mind 

 what sort of a creature an Entomostracan is. 



Water-fleas can boast of a very ancient ancestry, for they have 

 been traced back at least to carboniferous times, and seem to have 

 been as numerous in the standing pools of those vast forests which 

 produced our coal as they are now. The covering or carapace of 

 these lowly Crustaceans is very similar in composition to the 

 substance called chitine, which constitutes the outer portions of 

 true insects. The head of Daphnia and the animals grouped with 

 it as Cladocera is not covered by the carapace or so-called shell, 

 although in some entomostracans it is included, and the creature 

 has but one eye — a bright, inquisitive-looking organ, and evidently 

 quite capable of doing duty for tw^o. It is, in fact, a cluster of 

 eyes. In the Entomostracan world the females are in the major- 

 ity, and can consequently afford to be very merciful to the 

 numerically weaker sex. 



DapJmia possesses a sort of heart, a contractile organ, whose 

 workings can easily be seen. Generally there are five pairs of 

 legs. There are also two pairs of antennae, the larger pair being 

 used as natatory limbs. 



