236 MYRIOPODA. 



which, in form and position, more nearly resembles the unpaired 

 egg-tooth met with in the Opiliones (p. 33), while the egg-tooth 

 of Geophilus corresponds in position to the paired structure described 

 in connection with the Araneae (p. 58). Structures functioning in 

 this same way thus occur in the Arthropod a in very different 

 positions. 



The egg-tooth is, in any case, cast off later with the larval cuticle. 

 In the embryos of Polydesmus, which otherwise closely resemble 

 those of Strongylosoma, the egg-tooth is wanting (Fig. 118), no doubt 

 because the egg-integument in this form is much thinner, and a 

 special organ for splitting it is thus rendered unnecessary (Metsch- 

 nikoff). Julus also has no egg-tooth. 



The larvae of Polydesmus and Strongylosoma in a similar way 

 remain (though for a very short time) in the embryonic envelope 

 (Figs. 118 and 121), and are thus also at first incapable of free 

 movement. The pupal stage of Polyxenus, on the contrary, could 

 not be discovered, and Glomeris also does not seem to pass through 

 such a stage (vom Rath). 



It need hardly be pointed out how strongly the blastodermic 

 cuticle of the Crustacea, and still more the deutovum-membrane 

 of the Acarina, are recalled by the cuticular integument of the 

 Myriopod embryo. The resemblance to the deutovum-membrane 

 is increased by the discovery in Polyxenus of free amoeboid cells, 

 like the haemamoebae of the Acarina (Fig. 53, p. 99), outside 

 the embryo and between it and the egg-integument (or the cuticle, 

 where this is present) (Metschxikoff). "We must, however, regard 

 this merely as an analogous condition. 



Post-embryonic Development. 



Stages of post-embryonic development are represented even while 

 the embryo is still enclosed within the cuticular envelope, for the 

 embryo in many cases leaves the egg surrounded by this integument, 

 and must therefore already be regarded as a larva, and the envelope 

 as a larval integument (Fig. 121, p. 235). It has already been 

 mentioned that the so-called larva of the Diplopoda, apart from 

 the small number of its segments, does not differ greatly in form 

 from the adult. The possession of three pairs of legs brings about 

 a striking resemblance to an Insect larva ; vom Rath points out 

 especially its resemblance to the young Podurid. This is of course 

 merely an external resemblance, for, in the first place, the homology 

 of the cephalic regions of the Insecta and the Myriopoda (in respect 



