GENERATION. 131 



round embryo. The embryo lengthens, and the epithelium 

 disappears, except in the belt-like parts of the two extremi- 

 ties. (Siebold.) 



In Autolytus, the egg undergoing segmentation produces a 

 sexless larva (parent stock) which resembles the female in 

 general outline. Successive pairs of males and females are 

 developed from the posterior extremity of the parent stock; 

 the exact number has not been ascertained. (A. Agassiz.) 



Crustacea. Embryos are often dissimilar from the adult 

 form. The most conspicuous of these is the larva (Zoe) of 

 the Decapoda, and the active zoeform young of Cirripedia 

 (barnacles) and Lermeacea (lerneans). The latter groups are 

 inclusive of animals with sexual forms so degraded that their 

 position among articulates can be determined only by observ- 

 ing their larval conditions. 



Myriapoda. The larval form differs from the adult chiefly 

 in possessing a smaller number of segments. 



Arachnida. The young of certain Acarina (ticks) have 

 but six legs, as in Trombidium. 



Insecta. The metamorphoses of insects are various, 

 though fixed within natural groups. Some, however, are 

 without proper metamorphosis, as in Pcdiculus (louse), and 

 JLepisr.ia (silver-moth). 



Metamorphosis is incomplete when the larval condition 

 closely resembles the parent, as in Ilemiptera (bug) and 

 Orthoptera (locust). It is complete when the embryo passes 

 through two distinct stages prior to assuming the sexual form. 

 The first of these is the larva proper (caterpillar); the second, 

 the pupa (chrysalis). The sexual, commonly winged form, is 

 the imago. Examples are seen in Diptera (fly), Lepidoptera 

 (butterfly), Hymcnoptera (bee), Neuroptera (dragonfly), and 

 Coleoptera (beetle). The time required for the completion 

 of these life-phases is subject to variation. Thus in Masca 

 vomiloria (meat-fly) the entire development is completed 

 within thirty-one days, while in Cicada seplcndecim (seven- 

 teen-year locust) seventeen years are required. The quantity 

 and quality of food consumed exercises great influence upon 

 the embryo hastening or retarding development as it is 



