410 CEUSTACEA. 



joints of the second pair of antennae. On looking carefully, in this 

 situation the student will find a prominent tubercle formed by the 

 shell, the top of which is perforated by a small circular opening covered 

 with a tense membrane. Behind this orifice is placed a minute vesicle 

 filled with fluid, upon which a delicate branch of the antennary nerve is 

 distributed. This constitutes the whole apparatus : the vibration of the 

 water strikes upon the external membrane, the water in the sacculus 

 participates in the tremor, and the expanded nerve conveys to the brain 

 the sensation thus produced. 



(1048.) The function of this organ in the Lobster is contested by Dr. 

 Arthur Farre, who observes that it is situated not far from the mouth, 

 and is directed downwards ; it is by far the most sensitive part of the 

 body, since, while the mechanical irritation of any other parts excites 

 only slight movements in the limbs of the animal, the touching of this 

 part is immediately followed by violent and almost spasmodic flappings 

 of the tail. These circumstances, together with the situation of the 

 organ, appear to Dr. Farre to point it out as intended possibly for the 

 purpose of testing the quality of the food as, in fact, an organ of smell, 

 evidently endowed with an exquisite sensibility*. This, however, is 

 evidently merely a matter of conjecture, more especially as in the gene- 

 rality of the Crustaceans such an apparatus is altogether wanting. 



(1049.) The true organ of hearing, according to Dr. Farre, is situated 

 in the base or first joint of the lesser pair of antennae its precise seat 

 being indicated externally by a tough membrane, which covers an oval 

 aperture in the upper surface of this joint. Towards the inner and 

 anterior margin of this membrane there exists a small round aperture, 

 into which a bristle can be easily passed. On removing the membrane, 

 together with a portion of the surrounding shell, the internal organ is 

 brought into view, completely imbedded in the muscular structure of the 

 antennae. 



(1050.) This organ, the vestibular sac, nearly fills the cavity of the 

 joint, is somewhat sacciform in its shape ; and its walls present a delicate 

 horny structure of the consistence of a thin quill, being so transparent 

 as to admit of its contents being seen through the parietes. These are 

 found to consist of numerous minute particles of siliceous sand, which are 

 loosely contained in the interior of the sacculus. The walls of the ves- 

 tibular sac are furnished with several rows of minute ciliated processes 

 which, when highly magnified, are seen to be hollow and to be covered 

 with a fine down of hairs of exquisite delicacy, while in their interior 

 are contained numerous minute granules, which are apparently nerve- 

 granules. These processes are dilated at their base so as to form a 

 globular swelling, where they are articulated to corresponding circular 

 apertures in the walls of the sac, from which they spring in immediate 



* " On the Organ of Hearing in Crustaceans," by Dr. Arthur Farre, Phil. Trans. 

 1843, p. 234. 



