240 DR. ARTHUR FARRE ON THE ORGAN OF HEARING IN CRUSTACEA. 
touch, namely the antenna. And it is remarkable that the organ of hearing, as thus 
constituted, seems to be in its essential features no other than a delicate series of 
antennae, for the very form of the antenna with its marginal fringe of hairs is re- 
peated in the ciliated processes of the internal organ, but with this difference, that 
the latter are infinitely more minute, and therefore adapted to receive vibrations so 
delicate as to be inappreciable to the former. 
I feel that to carry the argument further would be to enter the region of specula- 
tion in regard to the precise nature of the sense to which this organ is appropriated. 
I have rather assumed, in common with others, that its office is that of ordinary 
hearing, from the close analogies which it presents with the elementary forms of that 
organ in other classes, and also from the generally received opinion that the Crus- 
tacea are highly sensitive of sound. But it is obvious that, if an organ of hearing, it 
constitutes at the same time but a repetition, upon a most refined scale, of the form 
of an organ of touch ; and it is difficult to refrain from hazarding a conjecture that a 
more extended and minute observation of the apparatus of the senses in different 
classes of the animal kingdom might develope other structures in which a similar 
approximation is made in their essential forms, as well as in the mode in which they 
are impressed by external agents ; and that while, as in this instance, an organism 
devoted to one sense may constitute but a repetition on a more refined and delicate 
scale of that of some other, the difference between the nature of the senses themselves 
may not be greater than is measured by this degree of approximation, and that the 
essential difference between their several organs may be found to lie in the degree of 
delicacy with which they are capable of distinguishing the vibrations of the media 
by which the entire organism is surrounded. 
The subject becomes more interesting in proportion as it is brought to bear upon 
the consideration of the means by which the descending series of animals, in whom 
the organs of sense become gradually diminished both in number and perfection of 
structure, until they appear to become merged in one single sense, are enabled to 
test the properties of surrounding objects. And the example therefore, as regards 
structure at least, of a kind of mixed sense, such as that now described, may it is 
hoped be not without its value in assisting us to arrive at a knowledge of the nature 
of the more obscure senses as enjoyed by the lower animals. 
Explanation of the Plates. 
PLATE IX. 
Fig. 1. Anterior portion of the body of a Lobster ( Astacus marinus) viewed from 
below. 
a a. The organ (of smell?) situated at the base of the second pair of 
antennae. 
b. Dilated base of the first pair of antennae containing the organ of hearing. 
