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EXPLANATION OF THE PLATES. 
The figures have been drawn either by ourselves, or under our immediate direc- 
tion and supervision in our laboratories. The colours are not diagrammatic, but represent 
accurately the natural conditions and the effects of differential staining or chemical 
reactions. 
Most of the histological work has been done with the Zeiss oil immersion 
and the Apochromatic 1.5 mm. and 2 mm. objectives, with the compensating ocular 12. 
Some of the figures have been drawn as seen under other powers, as indicated in the 
explanations. 
PLATE I. 
ILLUSTRATING THE GILL. 
Fig. I. — Diagram of the Anatomy of the Oyster (Osirea virginica, Gm.) from the left side, natural size, 
A. anterior ; P, posterior ; Z>. dorsal ; V. ventral ; a. anus ; add. and a/M.' the two parts of 
the adductor muscle ; aw. auricles of heart ; br. the two pairs of gills ; e.g. cerebral ganglion ; 
cr.s. crystalline style in first portion of intestine; h.d. one of the hepatic ducts opening into 
stomach ; i. intestine ; i.L the narrow intestinal loop ; I. liver ; lig. ligament ; m. mouth ; j^.p.i 
anterior and posterior labial palps ; p.m. end of reduced protractor pedis muscle ; p.h. pre-oral 
hood of mantle or cucullus ; paJl. pallium or mantle ; }•. rectum ; sh. shell ; st. stomach ; 
u. umbo of shell ; u.g.n. urino-genital aperture ; v. ventricle of heart, in pericardium ; v.g. 
visceral ganglion. 
Fig. 2.— Transverse section of part of a single branchia, to show its two lamella, their closely folded 
condition, and the large skeletal bars ( x 40). 
Fig. 3. — Transverse section across two crests of a single branchia, to show the gill filaments, an inter- 
lamellar junction, and the arrangement of the water and blood spaces. (Leitz, obj. 7). 
Fig. 4.— A few gill filaments from one lamella, seen in section, to show their histology. (Zeiss, obj. x\). 
Fig. 5. — A few similar gill filaments from a Dutch green oyster (Nieuport, Jan. 19, 1897), to show the 
distribution of the eosinophilous macroblasts. (Zeiss, obj. i'^). 
Fig. 6.— Piece of living gill of pale green "Native" oyster from West Mersea (Feb. 25, 1896), to show 
the active cilia and the green macroblasts. (Swift, i inch obj.) 
Fig. 7. — The green macroblasts on three adjacent gill filaments. (Swift, | inch obj.) 
Fig. 8. —Row of macroblasts seen in profile. (Swift, ^ inch obj.) 
Fig. 9.— Two adjacent gill filaments to show macroblasts (»?) and granular leucocytes (/). (Zeiss, iV oil 
immersion). 
Fig. 10. — A macroblast and two adjacent epithelial cells. (Zeiss, apochromatic.) 
Fig II. — Blood-cells from gill (Zeiss, apochromatic 1.5 mm.) 
Fig. 12.— Piece of free edge of gill of American oyster (June 8, 1898) showing leucocytes aggregated 
in numbers. 
Fig. i3.--Another place on edge of gill showing the leucocytes migrating out. (Swift, ^ inch obj.) 
