( 7 ) 
IV.— THE STRUCTURE OF THE BRANCHI^.. 
(See Plate I.) 
Figures 2 to 5 on Plate I. show the normal structure ot the gill in a healthy 
oyster, whether white or green. The oyster has the four great flattened branchial folds 
(PI. I., Fig. r, br), an external and an internal on each side, characteristic of Lamelli- 
branch Molluscs. On account, however, of the absence of the foot, the two internal 
branchiae come into close proximity in the middle line along their whole length, and so 
all four gills lie closely superposed along the ventral and posterior edges of the body : 
they encircle about two-thirds of the circumference. The branchiae occupy about one- 
third of the superficial area exposed on removal of a shell, and make up from one-third 
to one-sixth of the total bulk of the body. Physiologically they are one of the most 
important and constantly active organs of the body, as respiration is certainly not their 
sole, probably not their chief, function. They cause, by their ciliated epithelium, that 
powerful and all important current of water which carries food as well as oxygen into 
the body and removes all waste products. 
The gill filaments constituting the gills hang side by side ; but even those of 
one gill lamella, or surface, are not all in the same vertical plane, as the row of 
filaments is plicated to form alternate crests and troughs (PI. I., Fig. 2). The crests 
or projections form ridges visible to the eye, of which there are about one hundred 
or rather more on the surface of a gill ; and as each such crest has at least 10 (some- 
times 15) gill filaments, there must be over 1,000 filaments on each surface of each 
gill, or, say, 8,000 to 10,000 in all. The transverse sections shown in Figures 2 and 3 
show clearly the arrangement of the filaments in projecting crests on both surfaces of 
each gill. Concrescence (or inter-filamentar junctions) takes place to various degrees 
between neighbouring filaments, and consequently the blood channels form a network 
of varying arrangement in different parts, and the water passages are in many places 
partially occluded. In Figure 3, a indicates a spot where the filaments retain their 
primitive independence ; while b shows complete concrescence between the 12 filaments 
of one crest. This Figure and the other transverse sections show also the arrangement 
of the skeletal elements in the connective tissue of the filaments. Each filament has 
two longitudinally running skeletal bars, formed of condensed fibrillated connective 
tissue, roughly fusiform in section, and lying just under the epithelium on the external 
face of the filament (Figs. 4 and 5). At the bottom of each trough we find a single 
enlarged filament, with a modified pair of skeletal bars of much greater size, and of 
triangular form in cross-section. These pairs of larger bars in each trough are con- 
spicuous objects in low-power views {see Figs. 2, 3). 
