HABITS AND STRUCTURE OF ARENICOLA MARINA. 41 



long, seen from the dorsal surface, to show the gonidial vessel {G. V.), the 

 commencing processes of the dorsal lip, and the position of the external 

 opening (No.^) with regard to the neuropodium {Nnv".). The ventral lip 

 of the nephrostome {V.NpIi^.) is seen through the dorsal one. x CO. 



Fig. 17.— Funnel of the first left nephridium from the same specimen as 

 fig. 16, seen from the right side, to show the vertical position of the nephro- 

 stome and tiie commencing processes on the anterior lip. x 90. 



PLATE 4. 



Fig. 18. — The second right nephridium from a specimen 44 mm. long. 

 Dorsal view, to show the remarkably complete capillary circulation and the 

 extension of the vessel of the dorsal lip to form the gonidial vessel {0. F.). 

 Tiie ventral lip (V.Nph^.) is seen by transparency. X 65. 



Fig. 19. — Three views of the anterior end of a specimen 8 inches long 

 (littoral variety), to show the prostomiuni, nuchal organ, openings of the 

 otocysts, and the secondary annulation of the skin. a. From the left side. 

 B. From above, c. From below, x 12. 



Fig. 20. — Transverse section across the middle of the prostomium to show 

 the brain, the three prostomial lobes, and the rich blood-supply of this region. 

 The brain lies in the central prostomial lobe, and its covering of ganglion- 

 cells is closely applied to the overlying sensory epithelium. The section is 

 cut across in the region of the posterior cerebral cornua (P. Cr.). X 65. 



Fig. 21. — Transverse section of the same series as Fig. 20, across the 

 nuchal organ, Nu., the hinder cornua of the brain, and one otocyst, with its 

 contained otoliths. X 65. 



Fig. 22. — Transverse section of the body a short distance behind the third 

 diaphragm at the level of the openings of the oesophageal pouches {GL. Op.). 

 The external aperture of the second nephridium is shown on the right side. 

 The subdivision of the body-cavity into three longitudinal portions, and the 

 structure of the oesophageal pouches, are well seen, x 38. 



Fig. 23. — Transverse section of the body in the branchial region at the 

 level of a parapodium. The neuropodium is cut through its entire length on 

 the left side. On one side of the nerve-cord a retractor muscle from the 

 notopodium arises, on the other an oblique muscle. The vascular supply of 

 the body-wall, setse, and gills is well seen, x 38. 



PLATE 5. 

 Fig. 24. — Amoeboid and spindle-shaped cells of the coelom. x 1000. 

 Fig. 25. — Sagittal section of the brain slightly to the left of the middle 

 line, from a young littoral form about 3 inches long. The mass of ganglion- 



