PHYLUM ARTHROPODA 



301 



over the world by numerous species. A large species, D. magna, can 

 often be obtained from fish hatcheries or from tropical fish stores where 

 they are raised as fish food. 



The head of Dapfinia (Fig. 16.10) bears minute first antennae brist- 

 ling with chemoreceptors, and very large biramous second antennae, 

 which are locomotor organs. On the very rapid downstroke the an- 

 tennae are extended laterally, while on the slower upstroke the joints 

 bend, curving them close to the body. Behind the head and continuous 

 with it, the carapace extends posteriorly and ventrally to enclose the 

 rest of the body. 



Within the carapace are all the mouth parts and trunk limbs. Small, 

 blunt mandibles are followed by two pairs of minute maxillae and five 

 pairs of flattened biramous legs. The legs are used both for respiration 

 and for filtering microscopic food from the water. The last four body 

 segments bend ventrally and lack appendages. The body is made of six 

 head segments and nine trunk segments in all. 



By the beating of the trunk limbs and an intricate arrangement of 

 bristles, food filtered from the water is passed forward along the limbs 

 and pressed against the body behind the mouth. The mandibles chew 

 the front end of the food mass, pushing pieces of it into the mouth. A 

 short esophagus extends dorsally to open into the midgut, a long tube 

 that curves through the length of the body to a short rectum (hindgut) 

 and anus on the terminal segment. From the anterior end of the mid- 

 gut a pair of curved digestive pouches, comparable with the digestive 

 glands of the crayfish, extend into the head. 



A spacious hemocoel fills the body and limbs. Dorsally a portion 



^,.,._~^ |-P^a.-atenna. 

 Dig^estive 

 pouch. 



Esophagus 



Brood 

 pouch' 



Anu-S 



Midgut 



Figure 16.10. Daphnia, the water flea. Side view (left) with one side of the carapace 

 removed to show enclosed body and organs (modified from Lockhead). Ventral view 

 (right) with trunk appendages omitted. 



