EXPLANATION OF THE LETTERING. 



a Anus. 



b Branch of water-system, leading to dorsal pore. 



c Collar. 



d Dorsal pore. 



d! Central dorsal vessel. 



d!' Flat space between d' and gills. 



v d" Ventral folds between d f " and s. 



d' fl Central ventral vessel. 



d ly - Flat space between d! and genital organs, or alimentary canal. 



e Eye-specks. 



, f Folds of body-wall enclosing accumulations of mucus-secreting 



g Gills. 



g' Openings of gills leading outwardly. 



g" Skeleton supports of folds of gills. 



f Gill-folds. 



g xv - Openings of gills leading to oesophagus. 



h Heart. 



i Intestine. 



I 



Lateral flat part of the sides of the body, so-called lappets of Kowalevsky's Balanoglossus. 



lv Folds of liver. 



m Mouth. 



m b JVf uscular band running from eye-speck to anterior part of the water-system. 



o (Esophagus. 



p Skeleton at base of proboscis in Tornaria and young Balanoglossus. 



p' Proboscis. 



;>" Anterior opening of proboscis of Balanoglossus. 



p'" Posterior ventral opening of proboscis of Balanoglossus. 



q Genital organs. 



s Stomach or alimentary canal. 



w Upper lappet of posterior extremity of stomach in Tornaria. 



w r Lower lappet of posterior extremity of stomach in Tornaria. 



v Circular anal vibratile band of cilia. 



f Longitudinal undulating bands of cilia, 



w Water-system. 



w\ w' Right and left spurs of water-system. 



VOL. IX. 60 



