BY VERA A. IRWIN-SMITH. 



773 



in transverse and in longitudinal sections, due to variations in 

 its thickness. In places, it projects into the body-cavity in the 

 form of sharply angular thickenings (Text-fig. 20, s.c), but these 

 thickenings are not constant in position, and there is no indica- 

 tion whatever of definite longitudinal lines in it. No cell-bound- 

 aries and no nuclei are visible within it, but scattered nuclei 

 may occasionally be seen, lying along its inner surface (Text-fig. 

 \'2a). Four very definite thickenings of mesodermal tissue He 

 along the inner surface of this, and are respectively dorsal, 

 ventral, and lateral in position (Text-fig. 5, d.m.h., v.m.b., l.m.b.). 

 They extend throughout the length of the trunk, from the neck- 

 constriction to the beginning of the tail-region, forming four 

 longitudinal ridges on the body-wall (Text-figs. 1 2, 13). They 

 appear to consist of a clear, net-like protoplasm, containing 



Of" 



coel. 



Fif. 6^ FCp. 



Fig. 8. 



Figs. (5-8. — CJi. falcatum. 

 Tr. sees, through trunk of a young specimen, showing the four, large 



mesodermal ridges on the body-wall, and the structure of the 



alimentary canal; ( x 830). 

 Fig. 6. — Section through anterior region. 



Fig. 7. — Section through middle region, showing rudiment of genital S3^stem. 

 Fig. 8. — Section through region of the ventral setie. 



granules and nuclei, the latter being crowded along the inner 

 surface of the ridge. These ridges are most clearly distinguish- 

 able in a young and immature worm, where they are relatively 

 much larger than in the adult (Text-figs. 6-8). In a worm, in 

 which the alimentary canal and genital organs completely fill 

 out the coelomic cavity, and press closely against the body-wall 



