FRESHWATER LIFE. 35 



The latter are sometimes grouped together under the common designation 

 of Branchiopoda, or the gill-footed division. To that division we now 

 proceed. 



To the third order, the Cladocera, belong the various species of 

 Daphnia and its near relations. Let us first examine a full-grown 

 Daphnia, preferably one of the larger species, say D. puh-x or D. i^etula. 

 We see a body composed of two parts, the head teiininating below in a 

 sort of beak, and a thorax and abdomen, the two last enclosed in a 

 nearly transparent carapace. In D. pulex the carapace tapers off behind 

 in a long dagger-shaped point. In D. mucronata it bears two long spines 

 at the posterior corners. In D. vetula it is lopped off bluntly and slant- 

 ingly forwards. In D. reticulata it is marked with a network of lines 

 over its surface ; and in Acruperus harpce these lines resenrible the strings 

 of a harp. Below the beak are a pair of exceedingly small antennse, so 

 small that they may be easily overlooked. Springing, as it were, from 

 the neck, we see a pair of very large branched antennae. These are the 

 principal organs of locomotion. The eye consists of twenty crystalline 

 lenses, or fewer, and is turned about on a cushion of appropriate tissue 

 by two sets of muscles. This coarsely compound organ may be regarded 

 as foreshadowing the highly perfect compound eye of insects. Just 

 behind the eye may be seen the brain. At the junction of the head and 

 body near the base of the beak, is situated the mouth, which opens into 

 a short gullet, and that again into a roomy stomach with its two caRca ; 

 and then follows a straight intestine, which finally curves downwards- 

 towards the tail. Only the first segment of the body is adherent to the 

 carapace, the rest moving quite freely Vjetweeu the valves. Ample room 

 is thus afforded on the back of the animal for the accommodation of ita 

 eggs after they are laid ; and there, accordingly, the eggs are carried 

 about in a bundle till they are hatched. The tail, which terminates in 

 two hooks, is used as a rudder and propeller. Its motions are very 

 vigorous. If you look through the shell at that part of the body which 

 succeeds the neck, you will notice an oval-shaped organ, which keeps 

 regularly dilating and contracting. This is the heart. From its anterior 

 extremity springs an artery, and on opposite sides of the heart are two 

 slits which receive the blood from the surrounding cardiac chamber, and 

 close up at each contraction while the blood is urged forwards. There 

 are five pairs of legs, employed for other purposes than swimming. 

 When the animal is at rest, they create currents in the water, and so 

 bring food to the mouth. The first and second pairs are used as organs 

 of prehension. The third and fourth pairs mainly subsei-ve respiration, 

 for which they are well adapted by their branchial plates fringed with 

 numerous filaments. I have already remarked upon the extraordinary 

 fecundity of many of the Entomostraca. The Daphnise are no excep- 

 tions to the rule, and present similar phenomena of i^arthenogenesis. 

 The development of the embiyo may easily be watched through the thin 

 carapace, and is, of course, extremely interesting. The first organ to 

 show itself is the eye ; on the fifth day fi-om lading, the young DaphniaB 

 come out, and then go through their series of moultings. One great 

 advantage of the moulting process to the Entomostraca is, that they are 

 thus able to rid themselves of troublesome infusorial parasites, which 

 often lodge upon the carapace in great numbers, and seriously retard the 

 movements and depress the vitality of their host. Bunches of Epistylis 

 are very common upon Cyclops and Daphnia. Towards the approach of 

 winter, the Daphniffj have the remarkable habit of enveloping eggs in a 

 special casing between the outer and inner layer of the carapace. This 

 casing, which is developed on the back of the mother, has been called the 

 ephippium, from its fancied resemblance to a saddle. It wiU easily be 

 recognised by the microscopist. When the skin is cast, this structure ia 



