NATURAI, HISTORY OR AMERICAN EOBSTER. 
247 
THE BRANCHIAL CAVITY AND RESPIRATION. 
The branchise are lodged in a cavity of peculiar form upon either side of the body, 
where they are securely protected by the broad sides of the curving carapace. The 
gills (pi. xxxiv) arch upward in pyramidal form from the bases of the limbs and 
the sides of the body to which they conform, those of successive somites being divided 
by the gill separators or epipodites, which are hairy respiratory plates, springing from 
the basal segments of the limbs. Currents of water set upward and forward from under 
the free edges of the carapace, pass over the myriads of fine filamentous processes of 
branchiae, and are led into a trough or groove at the forward end of this curved narrow 
passageway on either side of the body. From this trough the water is fanned out by 
the rythmic beating movements of the “bailer” or respiratory plate of the modified 
second maxilla (see p. 228). The fan or respiratory paddle thus works with up-and- 
down strokes in a narrow passageway,® which is horizontal in front, and behind curves 
upward abruptly to the pyramidal apices of the gills. The lower bound of this passage 
is formed mainly by the epipodite of the first pair of maxillipeds, which is folded over 
so as to form a sort of trough in the part where the free inner division or epipodite of the 
bailer plays (pi. xxxvi, fig. 3 fd.). This fold presses against the side of the carapace and 
keeps water from entering the trough until it has passed over the lower half of the gills. 
The outgoing stream is thus essentially limited to the forward upper part of the gill cavity. 
By the alternate beating of the hinder (epipodite) and anterior (exopodite) divisions 
of the bailer the water is driven forward and out of the cavity. 
At the extreme hinder end of this chamber the carapace overlaps a small hairy leaf- 
like plate belonging to the fourteenth somite and bearing a small oval lacuna in its 
chitinous cuticle, just behind the pleurobranchia of this segment and above the hinge 
joint of the limb. This corresponds to similar lacunae for the four pleurobranchiae in 
front and without doubt represents the position of a former gill, every other vestige of 
which has now disappeared. 
As blood slowly passes through the 20 pairs of gills and their protective plates the 
act of respiration is accomplished. Carbon dioxide diffuses from the blood through the 
thin walls of the filament, and from the air dissolved in the sea water the oxygen supply 
of the blood is renewed. The water in the respiratory chamber is kept stirred up by 
the legs, to the bases of which 10 of the gills are attached, while the incessant 
beating of the fan at the front end of the cavity (marked by the frothing which 
commonly occurs when the animals are taken from the water) causes an active forward 
flow through the chamber and over the gills as described above. If the motion of the 
fan is stopped the animal soon becomes asphyxiated. The lobster will live for a long 
time out of water, in some cases for upward of two weeks, provided the branchiae are 
kept moist, and even in hot weather when the air is cooled by ice. 
From the filaments the aerated blood is conducted down one of the efferent branchial 
veins on the inner side of the stem in each gill, and thence through a distinct channel j 
one of the branchio-cardiac veins, to the heart. 
“The " fan” has been noticed to beat at the rate o( 95 to 178 strokes per minute in summer, in lobsters which had been 
out of the water long enough to become quiet. 
