VI 



CRUSTACEA 



261 



The body of the female Daphnia or Simocephalus (Fig. 109) is flattened 

 from side to side, and all except the head is enclosed between the flaps 

 of the carapace which forms a kind of bivalve shell. The abdomen, 

 which when at rest is bent forwards but which can be suddenly straight- 

 ened out, terminates in a pair of sharp claw-like extensions immediately 

 dorsal to which is the anal opening. Of appendages there are present 

 a pair of first antennae, reduced to small vestiges in the female ; large 



Fig. 109. 



Daphnia, adult female. X 40. a, Anus ; b.s, brood space between dorsal surface of abdomen 

 and carapace ; c.c, crystalline cone ; d.g, digestive glands ; E, radiate eye ; e, simple eye ; 

 em, embryos ; ent, intestine ; g, gill ; H, heart ; 0, ovary ; od, opening of oviduct ; oes, oesopiiagus ; 

 s.g, slieil-gland ; s.o.g, supra-oesophageal ganglion ; sp, spikes which prevent embryos from falling 

 out of brood space, in the natural position their points are close to the inner surface of the carapace ; 

 t.a., thoracic appendages. I, First antenna ; II, second antenna ; III, mandible. 



second antennae ; large [_- shaped mandibles ; small (first) maxillae ; 

 and five pairs of flattened thoracic limbs. The latter possess extensions 

 of their surface of two different types. Round their edges they extend 

 into closely set bristles which make them more efficient for driving a 

 respiratory current of water through the cavity enclosed by the carapace. 

 In addition each possesses a rounded thin-walled bulging of its surface 

 through which respiratory exchange takes place,, although no doubt this 

 takes place also through the entire inner surface of the carapace. In 

 the abdominal region the limbs have completely disappeared. 



