238 BUI.LETIN OF THE BUREAU OF FISHERIES. 



The lobster feels its way in the dark or gropes about in twilight by the aid of the 

 sensory hairs with which it is abundantly supplied. From 50,000 to 100,000 of these 

 organs are present on the big claws and slender legs alone. In most cases we do not 

 find it possible to discriminate between hairs which are solely tactile or for the chemical 

 sense alone. The lobster finds its way, however, to the fisherman's baited trap after dark 

 or in dim light by the aid of all those setae which respond to the chemical stimulus, and 

 chiefly no doubt by those on the anterior appendages, the hairs which project from 

 the lower sides of the outer whips of the antennules being probably the most sensitive. 

 Fine particles of the bait which difiFuse through the water from all sides of the trap, or 

 are carried by currents, furnish the stimulus which draws this animal to their source. 



BALANCING ORGANS OR STATOCYSTS. 



It is commonly observed that while a living fish swims with its body erect and poised, 

 a dead one floats on its side, and that the former position is one of unstable, and the 

 latter one of relatively stable equilibrium. The upright unstable position is maintained 

 in life by compensating moveme'nts which are automatically called into play by aid 

 of special sensory bodies called static organs. This is true of the lobster, and of all 

 animals which carry themselves upright, in opposition to the force of gravity. 



There is now considerable evidence to show that what were formerly regarded as 

 true "otocysts," or ear sacs, in the basal segments of the first pair of antennae, are 

 static rather than auditory in function, and accordingly they have been more appro- 

 priately called statocysts or organs of equilibration. The sac of either side (fig. 2) 

 fills nearly the entire segment, and is open to the outside by a fine pore barely large 

 enough to allow a minute grain of sand to pass, or to admit the point of a pin. The 

 membrane overlying this sac is thin and taut (fig. 4, pi. xxxv, mm.); long setae 

 encircle it, and also surround the mouth of the sac. 



The sac originates as a shallow pit of the skin, sinks into the tissues, becomes hori- 

 zontally flattened, and remains attached to the cuticle along its transverse front, the 

 opening being gradually constricted to a minute pore on the inner side of the thin 

 membrane. Upon dissection and examination of the sac from within, we see on its 

 floor a semicircular or horseshoe-shaped sensory ridge {s. r., fig. 3), studded with a 

 median row of about 75 plume-like hairs and four times as many shorter setae arranged 

 on either side or crowded about its mouth. Three hundred and seventy-five hairs 

 were present in a single case examined, but the number may be considerably greater. 

 Some of the hairs have bent shafts; some are thread-like, and scattered among them 

 and often glued to their tips are numerous fine sand grains, the "ear-stones" or otoliths, 

 as they have been called. In one of the sacs examined there were several hundred 

 grains, ranging from one-fortieth to one-six-hundredth inch in diameter, the smaller 

 being far too minute to be picked up with the points of the finest forceps. Each hair 

 of the sac is supplied with a nerve-element, and as Prentiss has shown, with but a single 

 one, as is the case with all tactile setae. 



