642 ZOOLOGY. 
nerve leading from them to the nerve-centres. In the 
clam it is to be looked for in the so-called foot. In the 
snails the auditory vesicles are placed in the head close to 
the brain, as also in cuttle-fish. The ears of Crustacea are 
sacs formed by inpushings of the integument filled with fluid, 
into which hairs project, and which contain grains of sand 
which have worked in from the outside, or concretions of 
lime. These are situated in the shrimps and crabs at the 
base of the inner antennae, but in certain other lower Crusta- 
cea, as in Mysis, they are placed at the base of the lobes 
of the tail. In the insects the ear is a sac covered by a 
tympanum, with a ganglionic cell within, leading by a 
slender nerve-fibre to a nerve-centre, and in these animals 
the distribution of ears is very arbitrary. In the locust they 
are situated at the base of the abdomen (Fig. 279) ; in the 
green grasshoppers or katydids and the crickets in the fore 
tibixs ; and it is probable that in the butterflies the antenne 
are organs both of hearing and of smell. 
The vertebrate ears are two in number and occupy a dis- 
tinct, permanent position in the skull, however much modi- 
fied the middle and outer ear become. 
Organs of Smell._The sense of smell is obscurely indi- 
cated by special organs in the invertebrate animals, nasal 
organs as such being characteristic of the skulled Vertebrates. 
Whether organs of smell exist in any worms or not is un- 
known ; there are certain pits in some worms which may 
possibly be adapted for detecting odors. In most insects at 
least the organs of smell are without doubt well developed ; 
the antenne of the burying beetles are large and knob-like, 
and evidently adapted for the detection of carrion. It is 
possible that certain organs situated at the base of the wings 
of the flies and on the caudal appendages of the cockroach 
and certain flies (Fig. 290) are of use in detecting odors. 
