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 sp. 



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. i 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. i 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. II.— 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 (;5r.) in widening the two 

 ciliated food avenues (see arrows) leading to the mouth (m.). 



PLATE III. 



ILLUSTRATING THE LIVER, &c. 

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



liver (L), pallial lobes (pall.), and the 4 branchise (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 (st.), 



intestine (^.^.), liver ducts (d.d.d.), and caeca of liver (c.c.c.) ; (Swift, obj. i 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. 8. — Transverse section of a well-formed liver caecum 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, t^i obj.). 

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



oyster (Zeiss, tV obj.). 

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



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



(Zeiss, apochr. 1.5 mm.). 



