I. CRUSTACEA 359 



rapidly in large numbers over suitable feeding grounds. Among the bees 

 parthenogenesis has a relation to the sexes, since males are only produced 

 from unfertilized eggs. Along with parthenogenesis there may be 

 rare exceptions sexual reproduction occurs, so that asexual alternates 

 with sexual generation (heterogony), though not in such a typical manner 

 as in the worms. 



Latreille divided the Arthropods into four classes: Crustacea, Myriapoda, 

 Arachnida, and Insecta. Later the discovery that Peripatus possesses trachea; 

 led to the creation of a new class, Protracheata, and the grouping of all arthro- 

 pods into Branchiata and Tracheata, 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 Crustacea 

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

 through different lines. 



Class I. Crustacea. 



The Crustacea are so called because 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 become 'crusty.' A 

 carapace, recalling the mantle of the molluscs, is widely distributed in the 

 Crustacea. It arises as a fold from the head, which may extend back- 

 wards as a shield, completely covering some or all of the thoracic seg- 

 ments (fig. 376), or it extends right and left on the sides of the body (fig. 

 389) and produces two valves strikingly like those of a lamellibranch, the 

 resemblance being strengthened in the cirripeds and ostracodes by the 

 extensive calcification. 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 occur where a rapid change of water is possible. 

 The appendages afford such a position, and hence one finds the gills as 

 thin-skinned vascular plumes or plates (figs. 62, 398) either on the appen- 

 dages or on the body near by; or the whole appendage may take a leaf- 

 like, thin-skinned shape and thus serve as a gill (fig. 375). Again, the 

 whole body surface may be respiratory and in small forms may entirely 

 replace the gills, so that these organs become rudimentary or entirely 

 disappear, there being a diffuse respiration with corresponding effects in 

 the circulatory system. With a localized respiration heart, arteries, 



