Z 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 
eis chain. On account of these 
correspondences and for other 
weighty reasons it is believed 
that arthropods have  de- 
scended from annelid-hke 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, Diplopoda, 
Chilopoda and Insecta. 
These classes are character- 
ized as follows: 
Crustacea.— Aquatic, as a 
rule. Head and thorax often united into a cephalothorax. 
Numerous paired appendages, typically biramous (Y-shaped) ; 
A scorpion, Buthus. Natural size. 
abdominal limbs often present. Two pairs of antenne. 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. 
