VI 



PROTRACHEATA 



435 



zone, throughout embryonic development, active processes of growth go on. Besides 

 this the cell material of the two mesoderm streaks themselves increases by a continuous 

 process of division. Segmented cavities appear in the mesoderm streaks in continuous 

 succession from before backward, and these separate into mesoderm segments or 

 mesoderm sacs with Avails, which are at first unilaminar. The further differentiation 

 of these mesoderm sacs occurs in such a way that each falls into three cavities, one 

 of which becomes the nephridial funnel (Fig. 299, A-C] Avhile the others disappear as 

 distinct cavities, and the cell material of their Avails yields the mesodermal por- 

 tions (endothelium, muscles, connective tissue) of the trunk and of the extremities. 

 The extremities arise as outgrowths of the body Avail. The first pair of rudimentary 



A 



FIG. 298. Embryo of Peripatus Edwardsii, 

 with growths beginning round the jaws. An- 

 terior end of the body from the ventral side (after 

 v. Kennel), fc, Jaws ; p, papillae, embracing the 

 jaws laterally ; op, oral papillae ; no, nephridial 

 aperture of the segment of the oral papillae. 



FIG. 299. A, B, C, Diagrams to elucidate the 

 development of the nephridia of Peripatus 

 Edwardsii (after v. Kennel). Only one side of 

 the body is represented in the transverse section. 

 I, II, III, The three divisions into which each 

 mesoderm sac falls ; II, the division which forms 

 the rudiment of the funnel. In A the rudiment 

 of the nephridial canal (nc) has appeared as an 

 invagination of the ectoderm, in B it has united 

 with the funnel rudiment (II) ; m, mesoderm ; Ib, 

 body cavity ; n, longitudinal trunks of the nervous 

 system ; d, intestine. 



extremities, after the antennse, develop into the jaws ; the second into the oral papillae 

 (Fig. 298). The two segments corresponding with them fuse with the primitive head 

 segment to form the later secondary head. 



The pharynx and oesophagus (stomodseum) and the hind-gut (proctodseum) form 

 by imaginations of the ectoderm which open later into the endodermal mid-gut. 

 The buccal cavity arises by the growing up of a rampart round the oral region, and 

 in this cavity the jaws come to lie. 



^^ T hile the coxal glands (including the slime glands), which proceed exclusively 

 from ectodermal invaginations, are clearly dermal glands, the nephridia (and the 

 salivary glands, genital ducts, and anal glands, which are homologous with- them) 

 arise out of paired rudiments. The funnel comes, as has already been mentioned, 

 from one part of a mesoderm sac and only later becomes connected with an ectodermal 

 invagination which yields the terminal vesicle, and, as it appears, the whole nephridial 



