328 THE INVERTEBRATA 



are differentiated in adaptation to different parts of the task. The third 

 and fourth pairs form a pumping and straining apparatus (Fig. 235) 

 which in principle is the same as those formed by the Umbs of 

 Chirocephalus ^ but has for side walls the carapace, against which the 

 proepipodites play, and is closed behind by a barrier formed by the 

 fifth pair. The broad exopodites of the third and fourth pairs open 

 and close the ventral side of the apparatus as they flap to and fro 

 under the pressure of the water. The long, feathered bristles of the 

 first and of the distal part of the second pair guard the ventral opening 

 of the median gully and keep too large particles from being drawn 

 into it. The complex set of bristles upon the large endite or "gnatho- 



pxh.'^ 



Fig. 235. A diagram of a transverse section through the thorax of Daphnia. 

 After Storch. hri. bristles of the fringes which strain out the food ; bri.' bristles 

 of the second pair of thoracic limbs which guard the opening of the median 

 gully; car. carapace; d. dorsal surface of the thorax; fd.gr. food groove; 

 me.gy. median gully or filter chamber; p. c/z. chambers between the limbs: the 

 enlargement and contraction of these chambers by the movements of the 

 limbs set up a pumping action by which water is caused to flow through the 

 bristle fringes from the median gully ; jpr.ep, proepipodites, playing upon the 

 carapace and closing the pumping chambers at their outer sides; th. 2-4, 

 sections through the thoracic limbs, which being directed backwards are cut 

 transversely : each limb underlies that behind it. 



base" (which is perhaps not the true gnathobase but the second 

 endite of the ideal series) in this limb play some part — exactly what 

 is disputed — in bringing the food to the mouth. Glands in the 

 labrum produce a sticky secretion as in Chirocephalus . 



The alimentary canal resembles that of Chirocephalus (p. 320), but 

 the coeca are unb ranched. The food on being swallowed passes direct 

 to the middle part of the mesenteron, where it is digested, and then 

 forwards to the anterior region and the coeca, where the digested 

 products are absorbed and the indigestible residue sent backwards to 

 be formed into faecal pellets in the hinder part of the mid gut. The 

 maxillary gland lies in the carapace. 



