NATURAI, HISTORY OF AMERICAN LOBSTER. 247 



THE BRANCHIAlv CAVITY AND RESPIRATION. 



The branchiae are lodged in a cavity of pecuUar 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 Umbs 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 maxilUpeds, 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 /d.). 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 gUls 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 channelj 

 one of the branchio-cardiac veins, to the heart. 



aThe " fan" has been noticed to beat at the rate of 95 to 178 strokes per minute in summer, in lobsters which had been 

 out of the water long enough to become quiet. 



