EXPLANATION OF THE PLATES. 57 



PLATE II. 



ILLUSTRATING THE REDUCED PROTRACTOR PEDIS MUSCLE, &c. 



Fig. I. — American oyster, with half of the left valve (c.s.) removed, to show the insertion of the reduced 

 pedal muscle at sj>. 



Fig. 2. — American oyster, with well-marked " spot " (sp.) formed by pedal muscle. 



Fig. 3. — Another specimen, with the edge of the spot strongly pigmented ( x 2). 



Fig. 4. — Section across the adductor impression of the shell ( x 2). The thin black line is dark brown 

 pigment. 



Fig. 5. — One end of a similar section magnified : a, prismatic layer ; b, opaque chalky layer ; c, trans- 

 parent columnar lining to adductor impression ; d, brown pigment (Swift, obj. 1 inch). 



Fig. 6. — Three sections across the impression formed on shell by insertion of pedal muscle. The smaller 

 black marks are brown pigment ( x 2). 



Fig. 7. — One end of a similar section magnified. Lettering as in Fig. 5 (Swift, obj. 1 inch). 



Fig. 8. — Epithelium on surface of mantle near " spot," for comparison with, 



Fig. 9. — Modification of surface of mantle at "spot" (Leitz, obj. 7). 



Fig. 10. — Section showing the fibres of the pedal muscle leaving the spot and running inwards and back- 

 wards (Leitz, obj. 3). 



Fig. 11. — Section further back, showing the muscle fibres coming into close relation with the large blood 

 sinus (lac.) and with the renal tubules. 



Fig. 12. — Section still further back, showing the fibres cut nearly transversely, and now lying below the 

 blood sinus and near the liver tubules. 



Fig. 13. — Last section of the series, showing the distribution of the muscle fibres along the top of the 

 anterior end of the external gill (all Leitz, obj. 3). 



Fig. 14. — Diagram to show the action of the modified protractor pedis muscle (pr.) in widening the two 

 ciliated food avenues (see arrozvs) leading to the mouth (;«.). 



PLATE III. 



ILLUSTRATING THE LIVER, &c. 



Fig. 1. — Oblique section of the visceral mass and gills of an oyster, to show relations of stomach (si.), 



liver (L), pallial lobes (pall.), and the 4 branchiae (br.). natural size. 

 Fig. 2. — Section of a visceral mass, to show a dark coloured liver, natural size. 

 Fig. 3. — Similar section showing a light yellow brown liver, natural size. 

 Fig. 4. — Low-power view of a section of visceral mass of a green Dutch oyster, to show stomach (at.), 



intestine (id.), liver ducts (d.d.d.), and caeca of liver (c.c.c.) ; (Swift, obj. 1 inch). 

 Fig. 5. — Section showing the opening of a liver duct into the stomach (Leitz, obj. 3). 

 Fig. 6. — Longitudinal section of a liver caecum (Leitz, obj. 7). 

 Fig. 7. — Transverse section of a liver caecum (Leitz, obj. 7). 



Fig. g. — Transverse section of a well-formed liver c;ecum of a green Dutch oyster (Leitz, obj. 7). 

 Fig. 9. — Three liver cells from the last, showing green granules in the cells (Zeiss, apochr. 1*5 mm.). 

 Fig. 10. — Section of a group of healthy, well-formed liver caeca (Zeiss, -rV obj.). 

 Fig. 11. — Section of a group of shrunken, degenerate liver caeca, from a sickly Fleetwood American 



oyster (Zeiss, ft obj.). 

 Fig. 12. — Hepatic cells (h.c), basal cells or " Fermentzellen ,: (b.c), connective tissue (c.t.), green 



leucocyte (/.), from liver caecum of green oyster with green granules (i/n. i/r.) in the liver cells. 



(Zeiss, apochr. 1.5 mm.). 



