6i3 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 ciittle-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 

 tibiae ; and it is probable that in the butterflies the antennae 

 are organs of hearing. 



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 some insects at 

 least the organs of smell are without doubt well developed ; 

 the antenniB 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. 



