This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1047). It is in the higher Crustacea that we, for the first time, indubitably find a distinct auditory apparatus; and, from the simplicity which the organ of hearing presents in this its earliest appearance, an inquiry concerning its structure becomes of great physiological interest. In the Lobster the ears are situated upon the under surface of the basal 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 generality 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 vestibular 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 apposition with a plexus of the auditory nerve, which has a separate and distinct origin from the supra-cesophageal ganglion.
* "On the Organ of Hearing in Crustaceans," by Dr. Arthur Farre, Phil. Trans. 1843, p. 234.
(1051). The existence of this singularly-constructed apparatus is by no means universal even among the Macrourous Decapods, and in the Brachyura it seems to be altogether wanting. We recognize, however, in its structure all the essential parts of an organ of hearing in its primitive form, viz. a distinct acoustic nerve, terminating in a plexus, which is expanded upon a vestibular sac. The remarkable arrangement of ciliated processes immediately overlying this plexus, with each process filled with nerve-granules, exhibits an apparatus for extending the extremities of the nerves in such a manner as to render them sensitive to the most delicate vibration of the fluid with which the sac is filled. But to heighten the effect of this, the grains of sand are added, thus forming adventitious otoliths, which, moving freely in the fluid contents of the sac, doubtless considerably increase the vibration of that fluid.
(1052). In the Brachyura, or Crabs, the membrane covering the external orifice of the ear is converted into a moveable calcareous lamella, from which, in some genera, a furcate process is continued internally; so that the whole, when removed by maceration, has no very distant resemblance to the stapes of the human ear, and, like it, seems to be acted upon by muscular fasciculi, so disposed as to regulate the tension of the vibratile membrane, and thus adapt it to receive impressions of variable intensity.
(1053). One of the first circumstances calculated to attract the notice of the anatomist who turns his attention to the structure of the generative system, both in male and female Decapod Crustacea, is the complete separation which exists between the organs belonging to the two sides of the body; for not only are the internal secreting viscera for the most part perfectly distinct from each other, but even the external sexual orifices are equally separate and unconnected.
(1054). Beginning with the parts observable in the male, we will take the Cray-fish (Astacus fluviatilis) as a standard of comparison, and briefly notice the principal variations from the type of structure observable in that species which are met with in other genera.
Fig. 207. Male generative apparatus of Astacus fluviatilis: a a, b, testicular mass; c c, vasa deferentia, forming by their convolutions a kind of epididymus, d d; f, their external orifices.