168 ON THE STKUCTURE AND 



latter does not contain corpuscles. With regard to their respira- 

 tion, Baird calls the beautiful comb-like organs attached to the 

 third and fourth pairs of feet, branchial plates. I think, however, 

 that this must be altogether a mistake, if it implies that respiration 

 takes place in them. They are surely unfitted for such a function, 

 the hard slender teeth of the combs are but ill-adapted to bring 

 the blood into contact with the external medium, and it is doubtful 

 whether the corpuscles could pass into them. The soft integument 

 of the pouches, where we see that circulation at least does 

 certainly take place, appears much better adapted for the purpose. 

 The combs may undoubtedly serve a subsidiary purpose by helping 

 to cause the influx of water through the valves, though that 

 would seem to be accomplished more by the action of the feet as 

 a whole, than by that of the combs alone, which I believe are 

 mainly instrumental in causing that accumulation of food 

 substances between the bases of the feet, which is the first step in 

 the act of feeding. Leydig considers, and I think with reason, 

 that the respiratory process is also largely carried on within the 

 valves, where a much larger circulation is maintained than seems 

 to be necessary simply for the reparation of their tissues. The 

 internal wall of the valves is delicate enough to subserve the 

 purpose, and its extent is all that could be desired. 



The nervous system, with the exception of the cephalic 

 ganglia, has, I believe, not been made out. The latter are seen 

 in the embryo to form a nearly continuous mass of nerve substance, 

 in the front of the head ; they subsequently become differentiated, 

 as follows : — A cephalic ganglion or brain (^r. Fig. ii) of a trian- 

 gular shape lies in front of the oesophagus, the apex extending to 

 the black spot (s), which has been supposed to represent the eye of 

 Cyclops, and the ocelli of insects. Posteriorly this is continued 

 as two nervous cords or crura, which embrace the labral muscle 

 and the oesophagus, beyond which it cannot be traced. A smaller 

 nerve mass, above this (o n), represents the united optic nerves of 

 the eye. From the rounded extremity of the latter, nervous cords 

 are given off to the several visual rods. A large ganglion (g), 

 connected by cords with the brain, is given off to the superior 

 antennae, and another (g) occupies part of the cavity of the 

 labrum, thus indicating that both these are sensory organs. 



The eye of Daphnia is one of the most interesting parts of 

 its organization. At first sight it seems to form an exception to 

 the usual form of the visual organs in the higher Crustacea, in two 

 important respects : firstly, that it is single ; and secondly, that it 

 is apparently immersed in the body cavity, and thus dissociated 

 from the epidermic tissues with which it is in other cases associ- 



