DESCRIPTION OF THE PLATES. 



c. The auricle of the left side. 



d. d. The two ventricles, left and right, the former shaded. Each ventricle 



gives off 



e. A single trunk which divides at once into two vessels, an anterior and 



posterior. 



h. Anterior aorta formed by the fusion of the two anterior vessels from 

 the left and right sides respectively. It passes forwards and supplies 

 the viscera, labial tentacles and anterior part of the mantle. 



g. Posterior aorta formed by the fusion of 



i. The two posterior vessels. 



j.j. The organ of Bojanus or nephridium. 



... FIG. 4. Longitudinal section, partly diagrammatic, of the left organ of Bojanus, or nephri- 

 dium of Unto pictorum. From de Lacaze-Duthiers, A. Sc. N. (4), iv. 1855, PI. V. Fig. 2. 



a. Posterior adductor muscle. 



b. Ventricle of heart traversed by the intestine, and giving off the anterior 



and posterior aortae. 



b'. b'. Pericardial space, immediately beneath the dorsal surface of the 

 animal. 



c. Rectal termination of intestine with anus. 



d. Dorsal edge of mantle in section. 



e. External aperture of nephridium. It leads into the duct, a thin-walled 



sac lying immediately below, i. e. ventral to, the pericardial space. 

 The two ducts, left and right, sometimes communicate beneath the 

 pericardial space anteriorly, e. g. in Anodonta. 



f. Internal aperture of the nephridium into the anterior extremity of the 



pericardial space. The arrow shows that the anterior part of the 

 glandular or lamellate portion of the organ passes internally to the 

 duct. 



g. Aperture of the generative organ, lying close to but separate from the 



nephridial duct in this mussel, and in the majority of Lamelli- 

 branchiata. 



h. The aperture from the non-glandular into the glandular portion of the 

 nephridium. The former has smooth walls, the latter walls produced 

 into lamellae. The cells covering these lamellae are pigmented and 

 contain phosphatic concretions. 



It is clear that water set in motion by the ciliated epithelium of the 

 organ can either enter or pass out of the pericardial space. 



FIG. 5. VeligersA Cardium pygmaeum before it quits the egg. From Loven, copied in Bronn, 

 Klassungen u. Ordnungen des Thierreichs, vol. iii. (i), 1862, PI. XXXVIII. Fig. 31. 



a. Entire circular velum, fringed with cilia, and bearing in its centre a 

 long flagellum. The velum, according to Loven, is formed by the 



