192 Aquatic Organisms 



segments consolidated on the dorsal side to form a hard 

 carapace, and have but five pairs of walking legs (as 

 the group name indicates), the foremost of these bear 

 ing large nipper-feet. This group contains the largest 

 Crustacea, including all the edible forms, such as crabs, 

 lobsters, shrimps, and pi'awns, most of which are marine. 

 Southward in the United States there are fresh-water 

 prawns (Palcemonetes) of some importance as fish food. 



The eggs of crawfishes are carried during incubation, 

 attached to the swimmerets of the abdomen, and the 

 young are of the form of the adult when hatched. They 

 cling for a time after hatching to the hairs of the swim- 

 merets by means of their little nipper-feet, and are 

 carried about by the mother crawfish. 



Crawfishes are mainly carnivorous, 

 their food being smaller animals, 

 dead or alive, and decomposing flesh. 

 In captivity they are readily fed on 

 scraps of meat. Southward, an omni- 

 vorous species is a great depredator 

 in newly planted fields of com and 

 cotton. Hankinson ('08) reports 

 that the crawfishes "forma very Fig- 99. Aseiiusaqua- 



. ,./- j,i 1 ■ r r If hcus, (x2, after Sars) 



important if not the chief food of 



black bass, rock bass, and perch" in Walnut Lake, 



Michigan. 



Spiders and Mites are nearly all terrestrial. Of the 

 true spiders there are but a few that frequent the water. 

 Such an one is shown in the initial cut on page 158. 

 This spider is conspicuous enough, running on the 

 surface of the water, or descending beneath, enveloped 

 in a film of air that shines like silver; but neither this 

 nor any other true spider is of so great importance in 

 the economy of the water as are many other animals 

 that are far less conspicuous. In habits these do not 

 differ materially from their terrestrial relatives. 



