ENTOMOLOGY 



arthropods have descended from annelid-like ancestors. Annelids, 

 however, as contrasted with arthropods, have segments that are essen- 

 tially alike, have no external skeleton and never have paired limbs that 

 are jointed. 



Classes of Arthropoda. Excluding the king-crab, trilobites and a 



few other forms that have no im- 

 mediate entomological importance, 

 the remaining arthropods fall into nine 

 classes, which are characterized as 

 follows : 



Crustacea. Aquatic, as a rule. 

 Head and thorax often united into a 

 cephalothorax. Numerous paired 

 appendages, typically biramous (Y- 

 shaped) ; abdominal limbs often pres- 

 ent. Two pairs of antennae. 

 Respiration branchial (by means of 

 gills) or cutaneous (directly through 

 the skin). The exoskeleton contains 

 carbonate and phosphate of .lime in 

 addition to chitin. Example, crayfish. 

 Arachnida. Terrestrial. Usually 

 two regions, cephalothorax and abdo- 

 men; though various Acarina have but 

 one and Solpugida have all three head, 

 thorax and abdomen. Cephalothorax 

 unsegmented, bearing two pairs of oral 

 Natural appendages and four pairs of legs. 

 Eyes simple. Abdomen segmented or 

 not, limbless. Respiration tracheal, by means of book-leaf tracheae, 

 tubular tracheae, or both; stigmata almost always abdominal, at 

 most four pairs. Heart abdominal in position. Example, Buthus 



(Fig. 2). 



Onychophora. Terrestrial. Vermiform (worm-like), unsegmented 

 externally. One pair of ringed antennae, a pair of jaws and a pair of 

 oral slime papillae. Legs numerous, paired, imperfectly segmented. 

 Respiration by means of short tubular tracheae, the stigmata of which 

 are scattered over the surface of the body, or arranged in rows. Genital 

 opening posterior. Numerous nephridia (excretory) are present, 

 arranged segmentally in pairs. Two separate longitudinal nerve cords, 



FIG. 2. A scorpion, Buthus. 

 size. 



