PLATE II 



1. Tornaria, seen from the dorsal side, showing first trace of change into Balanoglossus : the posterior part of 

 » the Tornaria greatly lengthened ; the longitudinal vibratile bands are reduced to indistinct lines of pig- 



ment-cells. 



2. Balanoglossus in a somewhat more advanced stage, seen from the ventral side. 



3. A different individual, nearly in state of Fig. 2, seen in profile. 



4. Somewhat older than Fig. 3, seen from the ventral side. 



5. Profile of a Balanoglossus, intermediate between Figs. 1 and 4. 



6. Balanoglossus considerably older than Fig. 4, seen from the dorsal side. 



7. Oldest Balanoglossus raised directly from Tornaria ; body has become greatly elongated ; the collar, proboscis, 



and posterior part of the body, are distinctly separated ; the anal band of vibratile cilia is scarcely ev*r 

 in movement. 



8. Youngest specimen of Balanoglossus dug up from the sand, magnified two diameters, in profile. 



9. The same magnified ; the proboscis, collar, and gills are like those of the oldest specimens found; the genital 



organs and liver have as yet not been formed. 

 10. Anterior portion of the same somewhat more magnified. 



11. 



con 



12. Anal portion of the same specimen, showing short intestine leading into the alimentary canal. 



13. Portion of gills, to show the folds and their mode of communicating externally, from a somewhat older 



specimen. 



14. Portion of the gills, to show mode of formation of the folds. 



15. Single gill-opening, leading to oesophagus. 



16. Skeleton at base of proboscis, seen from above. 



1 6 rt . Same, seen in profile : the forks are curved ventrally. 



17. Skeleton supporting the folds of one of the gill-openings. 



