24© BULLETIN OF THE BUREAU OF FISHERIES. 



It seems to be established that the supposed response of aquatic animals to atmos- 

 pheric sounds of ordinary intensity is a myth, for sound waves propagated in air are 

 almost totally reflected from the surface of water, but since sound vibrations are trans- 

 mitted by water it does not follow that aquatic animals are necessarily deaf. An animal 

 so abundantly supplied with tactile organs as a lobster has little need of ears, since sounds 

 transmitted through the water would be perceived or felt by means of the sensory hairs. 

 "The range of the average auditory organ in mammals," to quote from the work of 

 Prentiss, referred to above, " is from 30 to 16,000 vibrations per second; waves of less 

 than thirty vibrations per second do not usually produce auditory sensations, but are 

 appreciable to the tactile sense. It is important to note that decapods respond most 

 vigorously to low notes, and not at all to high notes or sounds produced by very rapid 

 vibrations. This fact would seem to be good evidence that the vibrations imparted to 

 the water and perceived by decapods correspond to those which produce tactile rather 

 than auditory sensations in vertebrates." It has been noticed that the so-called "audi- 

 tory" hairs of certain crustaceans will vibrate to different musical notes, as will the hairs 

 on the back of one's hand or the strings of a violin, but they are not auditory, as 

 Prentiss remarks. 



It is only natural to find that the senses of touch and hearing grade into each other, 

 and in either case it is the effect of a vibration which is perceived. While it is a matter of 

 convention how these sensations are described, it is evident that an aquatic animal like 

 the lobster has no organ strictly comparable to a vertebrate ear or even to the auditory 

 or chordotonal organ of insects, and that if possessed of such an instrument it would have 

 little occasion to use it. The basal segments of the large antennae of Palinurus possess 

 a peculiar structure often called a " stridulating organ," but nothing seems to be known 

 of the real uses which it serves. (See p. 160.) 



To return for a moment to the sacs, which have the form of a narrow-necked bottle, 

 and are carried in the antennulae, how do the sand grains find their way through their 

 minute openings, guarded with hairs ? Professor Brooks has seen the megalops larva of the 

 crab, Callinectes, pick up the grains and place them in the sac with its claws. As an 

 illustration of animal instinct, this is truly remarkable, for it is peculiar to the larvae alone, 

 the adult crab having no sand grains or otoliths of any kind in its sacs. The lobster at 

 the fourth stage nearly corresponds to the crab megalops, but it has never been seen to 

 behave in this manner. Whatever method the young may adopt to replenish their stock 

 of sand after each molt, it is evident from the microscopical proportions of the grains 

 that adults behave in a different manner. The animal in all probabiUty thrusts its head 

 in the sand, while the smaller grains, selected by the one opening of the "strainer," grad- 

 ally sift into the sac by the force of gravity. The spiny lobster (Palinurus), which also 

 keeps its antennal sacs well supplied with sand, has no claws with which to pick up any- 

 thing, and must have recourse to a similar method. In reference to this peculiar need 

 of the animal, it is interesting to notice that molting lobsters often burrow in the sand, 

 where they remain for some time after casting the shell. 



