408 ARTUROPODA. 



'Y\\Q Ciises of discoiclu] ;ind unequal segmentation are apparently 

 derived from tlie superficial. 



In accordance with tlieir liigli organization, reproduction by 

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

 esis do. Ill some parthenogenesis Jias a certain relationship to 

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

 it allows the sp)ecies to spread rapidly in large numbers over suit- 

 a.b]e feeding grounds. Ann:)ng the bees parthenogenesis has a 

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

 tilized eggs. Along with parthenogenesis — tliere may be rare ex- 

 ceptions — sexual reproduction occurs, so tliat not rarelj' asexual 

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

 a pronounced manner as in the worms. 



Tlie Fi'eiioli entomologist Latreille divided the Artljropods into four 

 classes: Ci'ustacea, Myriapoda, Anichnida, and Insecta. Later the dis- 

 covery, by Moseley, that I'eripiitus possesses tvachcce led to the creation of 

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

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

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

 trachete htive had ditferent 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. Siicilarly the inyriapods have been 

 divided, one gfoup, the ohilopods, 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 bard and firm by deposits of carbonate 

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

 has lost much of its elasticity ami has become ' crusty." Another 

 important characteristic is the htibittit of the group ; the Crustacea 

 ;ire typictilly acpiatic and hence breathe by nutans of gills. This 

 branchial res2)iration persists, as in the case of crayfish, when the 

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

 cba-mbcr and hence for a long tiirte the gills are wet bv this fluid. 

 There are l)ut few excc])tions to this rule, as some land crabs and 

 the sow litigs; those brctithe air, either by metins of the gills or by 

 special structures in the gill chamber to lie nrentioited later. 



The braindiia; or gills tire always placed where a. rapid exchange 

 of wtitcr is jiossible. The appenda.gcs afibrd such a position, ami 

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

 (ligs. I'll, 4.'!';') cither on the a.ppendages or on the bodv iu\ir by, 

 or tile whole ;tj)pendage may take a leaf-like, tlun-skiuned shape 



