PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 239 



reply that the larvse 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 Palasmonetes by lifting 

 the lid which covers the aperture, and forcing a fine jet of water into the 

 cavity. Most of the sand having been thus washed out, the animals 

 were placed in an aquarium upon the floor of which iron filiuo-s had been 

 scattered and were allowed to remain until the iron particles had been 

 takeu 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 tine copper wire, the termini beiuo- 

 connected with the circuit of a small six-celled battery. The shrimps 

 employed in the experiments (PaUemonetes) 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 tui'ued, 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 



