THE ARTHROPODA. 125 



fact that these forms are descended from ancestors which possessed 

 a richer segmentation of the body and a larger number of appendages. 

 The germ-band does not always retain its primitive position on the 

 surface of the egg, but may shift into the interior by undergoing a 

 ventral curvature (Myriopoda), or else, by the development of special 

 embryonic envelopes (amnion and serosa of the Insecta), it may sink 

 more deeply beneath the surface of the egg. A similar but merely 

 analogous development of embryonic envelopes is only found among 

 other Arthropoda in a few viviparous forms (Scoiyio and Peripatus 

 Edwardsii). [Cf. App. Lit. Insecta, Nos. XVI. and XLY.] 



The germ-band which until now corresponded merely to the ventral 

 portion of the embryo, spreads out over the lateral and dorsal parts 

 of the yolk which, so far, were only covered by a thin layer of cells, 

 these being now involved in the further shaping of the embryo, the 

 dorsal surface of which is thus produced. In the Insecta, these last 

 processes of development become complicated by the process of the 

 involution of the embryonic envelopes which takes place simul- 

 taneously. The closure of the dorsal body-wall completes the 

 external development of the embryo, which, after corresponding 

 further development of its internal organs, is ready for hatching. 



The newly-hatched embryo either resembles the adult, or else 

 differs from the latter, in which case it passes through a more or less 

 complicated metamorphosis. The process of metamorphosis diners 

 greatly in character in the different groups of the Arthropoda, but 

 must in all cases be regarded as the development of secondarily 

 acquired larval stages (Crustacea, Pantopoda, Insecta). The hatching 

 embryo either consists of merely a few segments (Crustacea, Pantopoda) 

 and only acquires the complete number of segments during the course 

 of metamorphosis (Diplopoda), or else has the full number of segments 

 as well as of body-regions possessed by the adult, from which it is 

 distinguished only by its different manner of life and by deviations 

 in the shape of the body determined by this life (Insecta). "We find, 

 for instance, that wings are wanting in all the larvae and young forms 

 of Insects, this characteristic of the most highly developed Arthro- 

 pods being thus proved to be a comparatively late acquisition, a view 

 which is confirmed by the fact that wings are still altogether wanting 

 in the lowest Insects (Apterygogenea, p. 260). 



One of the special characteristics of the metamorphosis of the 

 Arthropoda is the series of consecutive different larval stages which 

 pass one into the other through processes of ecdysis. Such moults 

 may also occur during embryonic life, and are even often found at 



