204 ONYCHOPHORA. 



merits of the jaws undergo considerable thickening of their outer 

 walls, the formation of the strong musculature of the jaws being 

 thus brought about, the inner wall supplies the cells which form the 

 muscles of the pharynx and stomodaeum. 



The primitive somites in the cephalic segment are at first very 

 large and occupy the greater part of the segment. As the ventral 

 organs and the brain increase in size, the primitive segments are, 

 however, pressed towards the dorsal surface, and thus become less 

 extensive. Parts of the primitive segments pass into the antennae 

 (as elsewhere into the feet), so that these latter at first appear to be 

 hollow, though the cavity degenerates later (Fig. 96 A, us). The 

 wall of the first primitive segment gives off cells for the formation 

 of the musculature of the oesophagus. According to Sedgwick, the 

 anterior primitive segment, like the rest, is divided into a dorsal and 

 a lateral portion, the significance of which will be discussed below 

 (cf. the Nephridia). 



The Musculature. 



Even in early stages, before any differentiation had taken place in the 

 primitive segments, cells became detached from them and became applied to 

 the ectoderm. These cells, and others which follow them during the further 

 development of the mesoderm, give rise, immediately below the ectoderm, to a 

 layer of circular muscle-fibres, which at first is thin, but in later stages becomes 

 much thicker (Sedgwick). The longitudinal muscles arise later than the 

 circular fibres, their fibres appearing in the cell-layer that covers the latter 

 internally. According to Sedgwick, they are distributed into various com- 

 plexes, one ventral, two ventro-lateral, two lateral and two dorsal, corresponding 

 to the longitudinal muscle-bundles of the adult. 



The musculature of the intestine and of the inner organs generally is derived 

 from the wandering cells which become detached from the primitive segments 

 and applied to these organs. 



The Nephridia. 



The nephridia arise in the following way from the lateral portions 

 of the primitive segments, the greater part of which occupy the 

 bases of the limbs. Each primitive segment has a conical outgrowth 

 directed towards the ventral side, which lengthens and, at the base 

 of the foot, fuses with the ectoderm, which becomes perforated at 

 this point, and thus the cavity of the primitive segment opens on to 

 the exterior (Fig. 101 A); this aperture persists as the external 

 opening of the nephridium (Sedgwick). The nephridium is now 

 essentially complete (Fig. 101), for it does not possess a funnel 

 opening into the adult body-cavity, as was formerly believed by 

 Balfour and Gaffron, but, according to the latest observations of 

 Sedgwick, throughout life ends blindly in this direction, the canal 



