CHAPTER VI. 



THE ARTHROPOD A. 



The segmentation of the body, that is, its division into 

 a series of somites, each provided with a pair of lateral ap- 

 pendages, which is so characteristic a feature of the higher 

 Annelids, is exhibited in a still more marked degree by the 

 Arthropoda. In these animals, moreover, the appendages, 

 themselves are usually divided into segments, while one or 

 more pairs of the appendages in the neighborhood of the 

 mouth are modified in form and position to subserve man- 

 ducation. Segmental organs, at least in their Annelidan 

 form, are wanting in the Arthropoda, and neither in the em- 

 bryonic nor the adult condition do they ever possess cilia. 



The process of yelk-division may be complete or incom- 

 plete, but no known Arthropod ovum gives rise to a vesicular 

 morula, nor is the alimentary cavity ordinarily formed by in- 

 vagination. 1 The precise mode of origin of the mesoblast 

 has yet to be worked out, but the perivisceral cavity appears 

 always to be developed by its splitting. In other words, it is 

 a schizocoele. 



As with Annelids, the segmentation of the body results 

 from the subdivision of the mesoblast by transverse constric- 

 tions into protosomites ; and there is every reason to believe 

 that the ganglionated nervous chain arises from an involution 

 of the epiblast. 



The neural face of the embryo is fashioned first, and its 

 anterior end terminates in two rounded expansions — the pro- 

 cephalic lobes — which are converted into the sides and front 

 of the head. The appendages are developed as paired out- 



1 The recent observations of Bobretzky on the development of Oniscus and 

 Astacus (Hofmann and Schwalbe, u Jahresberichte," Bd. ii., 1875), however, 

 tend to show that the hypoblast arises by a sort of modified invagination of 

 the primitive blastoderm. And in other Arthropoda there are indications of a 

 similar process. 



