6 



AN INTRODUCTION TO ENTOMOLOGY 



in the Arthropoda; the presence of paired ostia in the wall of the 

 heart; and the presence of a vascular body cavity and pericardium. 



They resemble the Annulata in having a pair of nephridia in most 

 of the segments of the body corresponding to the pairs of legs, and in 

 having cilia in the generative tracts. 



An extended monograph of the Onychophora was published by 

 Bouvier ('05-07). 



Class CRUSTACEA 

 The Crustaceans 



The members of this class are 

 aquatic arthropods, which breathe 

 by true gills. They have two 

 pairs of antennce and at least five 

 pairs of legs. The position of the 

 openings of the reproductive organs 

 varies greatly; hut as a rule they 

 are situated far forward. 



The most familiar examples 

 of the Crustacea are the cray- 

 fishes, the lobsters, the shrimps, 

 and the crabs. Cray-fishes (Fig. 

 7) abound in our brooks, and are 

 often improperly called crabs. 

 The lobsters, the shrimps, and 

 the true crabs live in salt 

 water. 

 Excepting Ltmulus, the sole living representative of the class 

 described next, the Crus- 

 tacea are distinguished 

 from all other arthro- 

 pods by their mode of 

 respiration, being the 

 only ones that breathe 

 by true gills. Many in- 

 sects live in water and 

 are furnished with gill- 

 like organs; but these 

 are either tracheal gills or 

 blood-gills, organs which 



differ essentially in struc- ^. „ ,,. , 



•^ Fig. 8. — Minute crustaceans: 



ture from true gills, as Cypridopsis, c, Cyclops. 



Fig. 7. — A cray-fish. 



Daphnia; 



