164 ON THE STRUCTURE AND 



epidermis, and blood corpuscles may be observed to circulate 

 within them. I believe they are the chief seat of the respiratory 

 process, though perhaps this is also carried on within the walls 

 of the carapace. The third and fourth limbs bear most beauti- 

 fully formed combs, — the branchial plates of Baird, — but this I 

 must regard as a misnomer. They are employed chiefly in 

 collecting the food into the gutter between the bases of the limbs, 

 which leads to the mouth. The abdomen is devoid of limbs, and 

 it is difficult to say, consequently, of how many somites it is 

 composed. It is bent upwards towards the head, and bears two 

 or three fleshy processes on its dorsal surface, one of which (/ r, 

 Fig. 4) is instrumental in keeping the eggs and embryos in their 

 places in the brood receptacle, and is terminated by two strong 

 hooks in front of the anus. 



The head of the Daphnia, though broad in the embryo, is 

 often very narrow in the adult, where it encloses the eye, but it 

 expands behind the bases of the antennae into a sort of hood 

 (/^, Figs. 2 and 3), which serves to protect the delicate cuticle at 

 the articulations. 



The whole of the body and limbs of the animal, with the 

 exception of the two pairs of antennae, are enclosed in the hard 

 cuticular covering of the head and carapace ; the valves of the 

 latter doubtless represent the branchio-stegites, or gill-coverings of 

 the higher Crustacea, and I believe also the wings of Insects. 

 Like these, they consist of a double wall (Figs. 7 and 8), and it is 

 within this double wall that much of the circulation of the blood, 

 which is so striking a feature in these creatures, goes on. It is 

 well to bear this in mind, as the impression so apt to be conveyed 

 at first sight, is that the stream of corpuscles carried round the 

 posterior margin of the valves, circulates between them, an 

 impression which a moment's reflection must show to be erroneous, 

 as in that case it would be exterior to the body. Near the 

 anterior margin of the valves there is a curious spiral marking 

 {s g^ Fig. 2), which Leydig calls the shell-gland, and likens it to 

 the green gland or renal organ of the crabs and lobsters. He 

 also says that the two walls of the valve are connected by 

 trabeculae, such as exist in the wing-cases of some beetles. 

 Deposits of lime sometimes occur within the valves; these are 

 of a somewhat stellate form (Fig. 10), and are affected by polarised 

 light. A deposit of pigment is also found in individuals of 

 advanced age. In Daphnia SchcBffcri I have found the animal to 

 be an opaque white from this cause. Sometimes, as I have found 

 it in D. psittacca^ the cavity of the valves is seen to be occupied 

 with cells containing granules. These cells are generally spherical, 



