140 NBRINB. 



internal. The hypodermic elevation in the mid-dorsal line forms a prominent ridge, 

 but it soon disappears. The alimentary canal with a deep involution dorsally fills up 

 the central region, and is slung by strong fibres from the mid-dorsal line. The 

 nerve-cords are widely separated, somewhat elliptical in section, and lie in the hypoderm 

 below the attachment of the oblique muscle on each side to the basement-layer, a pale 

 area occurring between them. They have thus gradually descended from the lateral 

 region. 



In this region a notable feature is the lateral position of the ventral longitudinal 

 muscles, for they are separated in the mid-ventral line by the extended inferior 

 attachment of the oblique, and by a small central group of longitudinal fibres above 

 them. The circular muscular coat passes externally from side to side under the 

 basement-membrane, and a strong band of the vertical muscle lies likewise at their inner 

 border, whilst other bands traverse the ventral longitudinal muscles. The great extent 

 of the ventral outline is thus accompanied by changes in the arrangement of the 

 body-wall. 



As soon as the body assumes a transversely elongated form the dorsal longitudinal 

 muscles are extended, and are, besides, intersected by the powerful vertical bands which 

 pass from the dorsal basement-layer to the ventral surface — through the ventral longi- 

 tudinal muscles. The oblique muscle on each side is more horizontal, extending from 

 the inferior bristle-bundle to the median line at the ventral surface, and across, or 

 even through the vertical bands, before insertion. The nerve- cords now lie close 

 together below the transverse muscle (a remnant of the circular), and a distinct neural 

 canal occurs at the inner and upper border of each. A few longitudinal fibres are still 

 present between the ventral attachments of the oblique muscles. Externally the ali- 

 mentary canal has its ccelomic epithelium, and thin layers of external longitudinal and 

 internal circular, the thick cellular mucous layer resting on a basement-layer. More- 

 over the canal is diminishing in area. 



It is soon apparent, in proceeding backward, that the vertical muscles descending 

 from the dorsal to the ventral surface do not interdigitate with the great longitudinal 

 muscles throughout their entire extent. They leave, as observed by the late M. 

 Claparede, at the external border of each dorsal muscle, a considerable mass, which bends 

 downward, and presents in transverse section a distinctly pinnate appearance, whilst the 

 rest of the muscle on each side is traversed by strong vertical fibres. These vertical 

 fibres which pass downward close to the wall of the alimentary canal are attached on 

 each side of the mid-ventral line (Fig. 113) to the incurved wall over the nerve- 

 trunks. The tips of the now slender oblique muscles still more nearly approach the 

 middle line, which is occupied by the ventral vessel and its branches, very few longi- 

 tudinal muscular fibres being apparently beneath them. The hypoderm in the 

 mid-ventral line is specially thickened, and the fused nerve-cords have a single large 

 neural canal in the middle, the nerve-area having internally the basement-tissue, circular 

 fibres, and longitudinal fibres and the ventral vessel. The fibres of the oblique muscle 

 enclose the area on each side for the ventral longitudinal muscle, which has both 

 an internal and external incurved process of muscle also distinctly pinnate in section. 

 The rest of this muscle is traversed by strong bands of the vertical muscle. The 



