FIG. 2. 



ENTOMOLOGY 



Furthermore, the sexes are almost invariably separate and the 

 primary sexual organs consist of a single pair. 



No animals but arthropods have all these characters, though 

 the segmented worms, or annelids, have some of them for 

 example the segmentation, dorsal heart and ventral nervous 



chain. On account of these 

 correspondences and for other 

 weighty reasons it is believed 

 that arthropods have de- 

 scended from annelid-like an- 

 cestors. 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. 

 Excepting the king-crab, tri- 

 lobites and a few other aber- 

 rant forms of uncertain posi- 

 tion, the members of the 

 series, or phylum, Arthropoda 

 fall into six distinct classes, 

 namely, Crustacea. Arach- 

 nida, Malacopoda, Diplopocla, 

 Chilopgda and I n s e c t a . 

 These classes are characteT- 

 ized 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 present. Two pairs of antennae. Res- 

 piration branchial (by means of gills) or cutaneous (directly 

 through the skin). The exoskeleton contains carbonate and 

 phosphate of lime in addition to chitin. Example, cray- 

 fish. 



A scorpion, Buthus. Natural size. 



