246 



MYRIOPODA. 



The difference in the formation of the enteron in the Chilopdda and in the 

 Diplopoda is very striking, and can best be seen by a comparison of Figs. 124 

 and 125. "While its formation in the Chilopoda^recalls the conditions found 

 prevailing in the Arachnida, in the Diplopoda its origin may be compared with 

 certain phenomena in the Crustacea, in which the epithelium also at first lies 



within the yolk until this 

 latter niters through into 

 the rudiment of the intes- 

 tine. Before these points 

 can be judged with any 

 certainty, further details 

 as to the later relations 

 of the enteron to the 

 nutritive yolk must be 

 ascertained. For the pre- 

 sent the whole subject is 

 obscure. 



The Stomodaeum 



and the 



Proctodaeum. 



The formation of the 

 oesophagus and the rec- 

 tum takes place in some- 

 what the same manner 

 in the Chilopoda and the 

 Diplopoda. Both these 

 „J- .^^^k^r^XJ^^^J^&^^VI:'^^ structures are ectodermal 



invaginations. I he sto- 

 modaeum arises ventral ly 

 between the cephalic 

 lobes, the proctodaeum 

 also lies ventrally near 

 the posterior end (Fig. 

 125, m and a). Each 

 lengthens, the former 

 posteriorly, the latter 

 anteriorly, and they 

 finally join the enteron, 

 with the epithelium of 

 which their walls fuse (Figs. 115, 124, 125). As the embryo grows, the anus 

 shifts from its former ventral position, and comes to lie further back (Figs. 

 110 to 112, pp. 224 and 225, and Fig. 124, a). 



The Malpighian vessels arise as two caecal outgrowths of the proctodaeum. In 

 Geophilus these apparently form very early (Fig. Ill, m.g), but in Julus their 

 rudiments appear only at the time of hatching {i.e., when the egg-shell splits, 

 on the twelfth day of development). 



The Mesodermal Structures. 

 In Julus the mesoderm first appears as an inwardly projecting keel- 

 shaped thickening of the blastoderm (Fig. 108, Heathcote), whereas, 



Fig. 126. — Transverse sections through embryos of Julus ter- 

 restris (somewhat diagrammatic, after Heathcote). A, the 

 ventral part of a transverse section through an embryo on 

 the eleventh day. B, complete transverse section of an 

 embryo on the twelfth day. Both sections are made through 

 the posterior region of the body. o;/, ventral ganglia ; d, 

 yolk ; dz, yolk-cells ; cd, proctodaeum ; ect, ectoderm ; ex, 

 rudiments of limbs; m.g, Malpighian vessels; p.lh, primary 

 body-cavity ; so, somatic, sp, splanchnic layer of the 

 mesoderm. 



