248 
BULLETIN OE THE BUREAU OF FISHERIES. 
COURSE OF THE BLOOD IN THE GILL. 
The description of the course of blood through the gill given above usually 
suffices for the text-books of zoology, but the physiologist wishes to know how the 
blood circulates in the gill filaments, for if these were simple capillary tubes it would 
tend to flow past rather than through them. The gill in reality is a complicated struc- 
ture, and the actual course of the blood is not easy to follow.® 
Each filament, like the stem of the branchia, is a double tube or vascular loop, 
consisting of outer afferent and inner efferent divisions (fig. 2, pi. xlvii.) All the blood 
must pass from the afferent branchial vein (af. v.) to the afferent divisions of the loops, 
thence to the efferent divisions, and then to the main efferent of the stem (ef. v.). The 
wall of the branchial afferent vein which carries unaerated blood to the filament sug- 
gests a cylindrical sieve or grater, with fine holes arranged in regular transverse rows. 
As the blood enters one of these holes it is conducted by a short passage to the afferent 
division of the loop or filament, but, as Dahlgren and Kepner have shown, the course by 
which the efferent half of the filament is reached is indirect. The venous blood in the 
afferent section enters a plexus of fine channels or capillaries, by which it is conducted 
around the filament and into the efferent loop. In the course of this passage the venous 
blood is brought close to the cuticular surface, but never quite touches it, there being 
always a cytoplasmic layer of the true epidermis of the filament, from which the cuticular 
covering is supplied at each successive molt. Thus, in passing through the filament the 
blood is kept in close relation to its surface, a condition which tends to promote the most 
active exchange of gases essential to respiration. These capillaries do not, apparently, 
have definite walls, but worm their way between or through the cells. The connective- 
tissue cells of the central core of the filament are described by Dahlgren and Kepner 
as being essentially peculiar and characteristic in possessing loosely branched proto- 
plasmic processes. The efferent channel of each filament empties into a circular vessel 
(fig. 1, pi. xlvii, c . v .) which runs around the main afferent of the stem, and thus 
conveys the arterialized blood to the efferent vein (ef. v.). 
The course of the blood through the gill is thus, in brief, as follows: Stem afferent 
to filament afferent, through filament capillaries to filament efferent, to circular vessel in 
wall of stem afferent , to stem efferent, to branchio-cardiac vein, to pericardium and heart. 
This system of vessels is filled with blood, which, owing to the rhythmic contractions 
of the heart and the dispositions of its valves, is kept moving in the same direction, 
from heart to tissues, from tissues to gills, and from gills to heart again. The heart is 
“arterial,” and the breathing organs of the crustacean are thus introduced into the 
returning stream of venous blood, the converse of the conditions found in fishes, where 
the heart is “venous” and the gills participate in the arterial system which leaves it. 
o The account of the circulation of blood in the gill given in this section was written six years ago, when the drawings illus- 
trating it were made. Certain details concerning the capillary plexus have been added since reading the work of Dahlgren and 
Kepner, who, so far as we are aware, were the first to describe the histology of the filament and the course of the blood through it. 
