ECONOMY OF THE DAPHNIA. 167 



previous to a moult, and that it is cast together with the external 

 cuticle of which it is the homologue. The contents of the 

 stomach, I have sometimes noticed, exude through it, as if it had 

 lost its continuity in places ; I have also seen a similar sloughing 

 condition of the internal membrane of the stomach of the 

 larva of the Crane Fly. The remaining portion of the ali- 

 mentary canal consists of a short rectum, or hind-gut, of consid- 

 erably less diameter than the stomach ; it opens immediately 

 below the large pair of anal hooks (see r, Fig. 4). A peristaltic 

 movement is visible in the stomach, a wave of contraction 

 passing forward along that organ. A pair of coeca {coe.^ Figs. 

 3, 4, and 11) are found in the head, they open into the stomach 

 at its commencement, just anterior to the great bend, and 

 represents the more complicated liver of the higher Crustacea. 

 Like the stomach they are furnished with muscular walls and a 

 glandular epithelium. A movement of alternate expansion and 

 contraction commingles their contents with those of the stomach. 



The circulation in Daphnia is entirely lacunar, there are no 

 such simple arteries even as those found in the Crayfish. The 

 heart is lodged in a special chamber, the pericardial sinus (/ s, 

 Fig. 4), immediately in front of the upper end of the brood 

 receptacle. Into this chamber the current of blood comes from 

 the dorsal margin of the valves, and enters the heart by two 

 lateral sHts (Fig. 12). When from any cause the pulsation of the 

 heart is retarded these slits may be seen to open and close 

 alternately. From the heart the circulation proceeds into the 

 head, bathing the great nervous centres, and passing into the 

 labrum ; from thence its course becomes much more obscure. A 

 strong current, however, circulates within the valves, /.<?., between 

 their double walls, and collecting at their dorsal margin, passes 

 thence back to the heart. In the abdomen also a strong current 

 is seen passing between the stomach and the body-wall towards 

 the heart, before reaching which it seems to encounter another 

 current coming from that organ ; this latter, however, I believe 

 separates on either side of the stomach and passes over toward 

 the feet ; currents are also seen in the pouches of the feet. 



The heart is stated by Leydig to beat at the rate of from 200 

 to 250 times in a minute. The circulation, I have sometimes 

 observed, is better seen in Daplmia vetula than in the other 

 species, the corpuscles being larger. The blood-plasma, usually 

 colourless, is under some circumstances found tinged with red to 

 such extent, as to impart a ruddy hue to the water in which the 

 creatures live ; it is then singularly like the red fluid circulating 

 in the closed vessels within the bodies of worms, except that the- 



