408 ABTHROPODA. 



The cases of discoidal and unequal segmentation are apparently 

 derived from the superficial. 



In accordance with their high organization, reproduction by 

 fission or budding never occurs, but parthenogenesis and pa?dogen- 

 esis do. In some parthenogenesis has a certain relationship to 

 the life history. In lower Crustacea and in Aphides (plant lice) 

 it allows the species to spread rapidly in large numbers over suit- 

 able feeding grounds. Among the bees parthenogenesis has a 

 relation to the sexes, since males are only produced from unfer- 

 tilized eggs. Along with parthenogenesis there may be rare ex- 

 ceptions sexual reproduction occurs, so that not rarely asexual 

 alternates with sexual generation (heterogony), though not in such 

 a pronounced manner as in the worms. 



The French entomologist Latreille divided the Arthropods into four 

 classes: Crustacea, Myriapoda, Arachnida, and Insecta. Later the dis- 

 covery, by Moseley, that Peripatus possesses tracheae led to the creation of 

 anew class, Protracheata, and the grouping of all arthropods into branchi- 

 ate and tracheate divisions, the branchiates including the Crustacea alone. 

 Later researches have shown that these divisions are not natural and that 

 tracheae have had different origins, the spiders being nearer to the crus- 

 tacea than to the insects, and that Crustacea and insecta have come from 

 the annelids through different lines. Similarly the myriapods have been 

 divided, one group, the chilopods, being closely related to the true insects, 

 the other (diplopods) being very uncertain in position. 



Class I. Crustacea. 



The Crustacea owe their name to the fact that their chitinous 

 cuticle is usually rendered hard and firm by deposits of carbonate 

 and phosphate of lime and, in contrast to that of other arthropods, 

 has lost much of its elasticity and has become ' crusty/ Another 

 important characteristic is the habitat of the group; the Crustacea 

 are typically aquatic and hence breathe by means of gills. This 

 branchial respiration persists, as in the case of crayfish, when the 

 animals are taken from the water, for they retain water in the gill 

 chamber and hence for a long time the gills are wet by this fluid. 

 There are but few exceptions to this rule, as some land crabs and 

 the sow bugs; these breathe air, either by means of the gills or by 

 special structures in the gill chamber to be mentioned later. 



The branchiae or gills are always placed where a rapid exchange 

 of water is possible. The appendages afford such a position, and 

 hence one finds the gills as thin-skinned vascular plumes or plates 

 (figs. 61, 437) either on the appendages or on the body near by, 

 or the whole appendage may take a leaf -like, thin-skinned shape 



