174 bulletin: museum of comparative zoology. 



To this is added a table, embracing all the forms which have been stud- 

 ied, giving the names of the different investigators, and the conditions, 

 as to number and size, of both the auditory hairs proper (Otolithen- 

 haare) and the "free auditory hairs " found on the antennae and abdomi- 

 nal appendages. 



Lemoine ('68) compares the otocyst of the lobster with that of the 

 crayfish. His descriptions are similar to those of Farre ('43), but his 

 figures are poor. The thin dorsal wail of the basal segment of the first 

 antenna, which covers the ear sac, he calls the tympanic membrane of 

 the lobster. The opening of the sac is overlooked, and the otocyst de- 

 scribed as closed. Thus, as the otoliths cannot come from without, 

 Lemoine's theory is that they are exfoliations from the calcified walls of 

 the sac, — an absurdly impossible assumption, as the thin chitinous walls 

 of the otocyst are not calcified. In the case of the crayfish, he notes 

 nothing new except that there is a membrane at the base of each hair 

 shaft, separating its cavity from that of the spherical enlargement on 

 which the shaft stands. This membrane acts as an ear drum, taking the 

 place of the large tympanic membrane described for the lobster. 



Garbini ('80) discusses very briefly and incompletely the sense organs 

 of Palsemonetes varians. The figures of the otocyst are extremely crude 

 considering the date of the work, and simply confirm the conditions found 

 by Hensen in Palaemon. 



Vom Rath ('87, '88, '91, '94) does not make the sharp distinction 

 between auditory and tactile hairs that Hensen does, holding that the 

 two kinds grade insensibly into each other, the auditory hairs being 

 simply slightly modified tactile organs. All sensory bristles of Crustacea 

 can be divided into two chief groups : — 



(1) Tactile or auditory hairs, with long, plumed shaft, the base of 

 which is attached to the body wall by a delicate membrane of chitin, 

 often spherical in form. Differentiation is thus towards freedom of 

 movement in response to tactile or vibratile stimuli ; (2) taste or olfac- 

 tory hairs, having a short blunt shaft, thick-walled at the base, but with 

 either a small pore or thin permeable membrane at its distal end, by 

 means of which chemical substances in solution can come into direct 

 contact with the nerve endings. The nervous apparatus of these hairs 

 is the same in both cases for all decapods. The sweeping statement is 

 made, that beneath every sense hair there lies, either in the hi/pndermis, 

 or removed some distance from it, a group of bijjolar ganglion cells. From 

 each of these cells a fibre is given off peripherally, and these, forming a 

 strand, enter the base of the hair, ending only at its very tip. 





