Gatty Marine Laboratory , St. Andrews. 57 



from the external mass, jor., throughout the reticulated central 

 region and their fusion with the inner edge are indicated. 

 in., muscle. X 650 diam. 



Fig. 11. Transverse section of the anterior region of Bisjnra. r/., oeso- 

 phagus surrounded by firm muscular bands ; tg., thoracic 

 glands with pale membranous tubes of chloragogenous tissue, 

 chl., attached; nc, great nerve-cords, x oc. 4, obj. A. . 



Fig. 12. Transverse section of two branchial filaments with their chordoid 

 axes before separation, x oc. 4, obj. D. 



Fig. 13. Transverse section of a ventral longitudinal muscle in the poste- 

 rior region of the annelid, with portions of a segmental organ, 

 so. X oc. 4, obj. A. 



Plate III. 



Fig. 14. Transverse section of the ganglionic region of Bisjnra vohita- 

 cornis, showing the eyes, oc. cp., last trace of the external 

 cephalic pit ; vf., ventral fimbriae, x oc. 4, obj. A. 



Fig. 15. Transverse section oF the ganglionic region with the great nerve- 

 cords, ?ic., at the sides of the oesophagus and about the level of 

 the chordoid skeleton, ch., the external margin of which is not 

 smooth, but has processes. Cells and fibres intervene between 

 the dorsal muscles, dm., and strong transverse fibres below 

 them, whilst under these are vertical fibres, more or less 

 mesenterial, in which the common duct of the thoracic glands, 

 tgo., lie. The massive ventral hypoderm, /yj., occurs inferiorly. 

 The dorsal region is only partially represented, and the lower 

 division of the great muscular mass is only indicated at 7/i. 

 The preparation is somewhat stretched inferiorly. Enlarged. 



Fig. 16. Transverse section toward the termination of the thoracic 

 glands, tg., which are represented by two tubes, om., con- 

 nective tissue with nuclei and muscular fibres, probably part 

 of the oblique muscle of the side, x oc. 4, obj. A. 



Fig. 17. Transverse section of the region of the nerve-cords in the middle 

 of the body, hp., hypoderm ; nc, nerve-cords ; vv., ventral 

 blood-vessel with a coating of chloragogenous cells, chl. 

 X oc. 4, obj. A. 



lig. 18. Slightly oblique section of a branchial filament of Pomatocerus 

 triqueter. bv., blood-vessel ; cce., coelomic space ; n., nerve. 

 X oc. 2, obj. D. 



Fig, 19. Transverse section of a branchial filament toward the base and 

 where its inner edge is produced into a groove with ciliated 

 sides. X oc. 2, obj. D. 



Plate IV. 



Fig. 20. Transverse section of the anterior region of Pomatocerus tri- 

 queter, L., near the origin of the opercular stalk (op.), d., the 

 vestibule ; ds., dorsal pit ; hpi., modified hypoderm covering 

 the inner surface of the thoracic collar or jacket and the outer 

 side of the body-wall. The dorsal surface is to the right, 

 X about 35 diam. 



Fig. 21. Transverse section of the body-wall in front of the foregoing. 

 The opercular stalk {op.) projects much further, the dorsal pit 

 [ds.) is larger, and the slits (bf.) indicating the spaces between 

 the branchial filaments are present. Spaces {cce.), apparently 

 coelomic, occui" on each side. X about 35 diam. 



