INTRODUCTION. 1x 
the eye than in the adult state. In the genus Gam- 
marus, the number of lenses in the young is first eight or 
ten, whilst in the adult they number from forty to fifty. 
The superior or first pair of antennze we consider, con- 
trary to the opinion of Mr. Dana, to be formed on the 
same type as those of the Macrura. Each of them con- 
sists of three distinctly formed joints and a flagellum, 
with sometimes a more or less important secondary 
appendage. We have long since expressed our opinion 
that in these organs lies the seat of auditory consciousness, 
and we are still inclined to retain that opmion. We are 
aware of the elaborate experiments of Dr. Von Hensen, 
which tend to demonstrate the existence of auditory cilia 
on several parts of the animal, as the superior antenna, 
(in which Professor Huxley was the first to demonstrate, 
in some exotic Macrura, the presence of highly refracting 
otolithes,) on the inferior antennze, as well on the caudal 
appendages as in the external branch of the posterior 
pleopoda, on which Van Beneden has discovered, and we 
have seen, what appear to be well-formed otolithes, of the 
same type as those found in the first joint of the anterior 
pair of antenne in Mysis, &c. But we have always 
attributed to certain very delicate membranous cilia of 
various forms, found on the primary flagellum only of the 
superior antenne, and present, under normal conditions, 
in nearly every family of Crustacea, the power of convey- 
ing impressions of sound. But these membranous cilia 
are very distinct from the auditory hairs of Dr. Von 
Hensen.* 
That the superior antenne are, in their most normal 
development, purely aquatic organs, we see in the depre- 
ciation of their character in the partly marine genera 
* An elaborate memoir on the auditory organs of the Crustacea, by Dr. V. 
Hensen, was published in Zeitschr. f. Wissensch. Zoologie, xiii. Bd. 3. Hft. 
1863, an abstract of which may be seen in the Zoological Record for 1864. 
