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PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 259 
reply that the larve were perfectly normal when observed, as far as 
feeding and active swimming were concerned, and furthermore that the 
loss of equilibration disappeared at once when a larva without otoliths 
was allowed to obtain them. The results of these observations are 
also confirmed by the following experiments. 
The otoliths were removed from the sacs of Palemonetes by lifting 
the lid which covers the aperture, and forcing a fine jet of water into the 
cavity. Most of the sand having Leen thus washed out, the animals 
were placed in an aquarium upon the floor of which iron filings had been 
scattered and were allowed to remain until the iron particles had been 
taken into the sac in place of grains of sand. As an electromagnet, a 
steel bar 8 inches long and one quarter of an inch square was used. 
This was ground down nearly to a point at one end; about the other 
end were wound many layers of fine copper wire, the termini being 
connected with the circuit of a small six-celled battery. The shrimps 
employed in the experiments (Palzmonetes) were blinded by the usual 
method, — painting the eyestalks with a mixture of lampblack and 
shellac. The pointed end of the magnet was held about 3 cm. from the 
otocysts, at one side of and a little ventral to them. Animals with 
normal otoliths, if blinded, do not respond at all, and are apparently 
unaffected by the proximity of the magnet; they keep their normal 
position, dorsal side up, with the sagittal plane of the body coincident 
with the direction of gravity. If not blinded, they simply move slowly 
away from the magnet when it approaches too near. When, however, 
the magnet is brought into close proximity to otocysts containing iron 
filings, the dorsal side of the animal is turned, not toward the magnet, 
as might be expected if the changed position were due directly to the 
action of the magnet on the iron filings, but away from it. If the 
magnet was changed to a position on the other side of the shrimp, the 
turning was in the opposite direction, still away from the source of 
attraction. 
The above reaction was distinctly noted a number of times for each of 
the six animals experimented upon. As Kreidl’s work was fairly com- 
plete, only one series of experiments was tried in confirmation of his 
results. When the observations had been completed, the antennules of 
the six shrimps were removed and the otocysts examined under the 
microscope. In each case particles of iron were found nearly filling 
the sac, and if a magnet was held close to one of the latter, the whole 
antennule was lifted by the attractive force, showing clearly that there 
must have been an effective magnetic pull upon the otoliths of the live 
