414 SPECIAL PHYSIOLOGY. 



auditory organ. In such cases, the sonorous vibrations must reach the laby- 

 rinth directly, through the framework of the head. The otoliths are more 

 largely developed in the simpler forms of auditory apparatus. 



In such of the non-vertebrated animals as are purely aquatic, the auditory 

 apparatus consists essentially of a sac or vestibule, in which a more or less 

 regularly formed cretaceous otolith is found, and which is connected with a 

 special nerve. Into this sac, the sonorous undulations are readily conveyed 

 from the water ; it is analogous to the membranous vestibule of the vertebrate 

 ear. In the air-breathing Annulosa, other contrivances, of which dry elastic 

 membranes, calculated to receive vibrations through the air, form a part, are 

 met with. 



In the Mollusca, double symmetrical organs of hearing are present in all the 

 classes, even in certain Lamellibranchiata. They are connected by means of 

 short auditory nerves, either with the suboesophageal ganglia, as in the higher 

 Mollusca, or with the pedal ganglia, as in the Gasteropodous and lower forms. 

 In the Cephalopoda, these organs consist of two nask-shaped sacs, the ana- 

 logues of the membranous labyrinth of the Yertebrata. They lie close together 

 in an excavation of the cartilage of the head, the cartilaginous vestibule. 

 Each contains a large cretaceous otolith, with some fluid. The space between 

 the sac and the cavity in which it is lodged is filled with gelatinous fluid, and 

 the cavity itself is perforated by the auditory nerve, which is distributed to 

 the sac. In the remaining classes of the Mollusca, the organs of hearing are 

 more simple, each consisting of a simple roundish or oval sac, situated in the 

 soft parts, closely attached to the auditory nerve, and containing a fluid, with 

 a central otolith^ suspended in it ; sometimes the sac is lined with a ciliated 

 epithelium. Amongst the Molluscoida, similar auditory vesicles are found in 

 some Ascidioida. 



The Annulosa are not universally provided with acoustic organs. In In- 

 sects, it has been conjectured, that such organs exist at the base of the an- 

 tennae, where a soft membrane, made tense by those parts, is supposed to 

 represent a sort of tympanic membrane ; others imagine that the antennae 

 themselves, being supplied with large nerves, can appreciate vibrations. In 

 the grasshopper and cricket tribes, there is sometimes found, on both sides of 

 the first abdominal ring, a large oblong depression, set in a firm horny ring, 

 and closed at the bottom by a delicate membrane. A little vesicle, containing 

 a watery fluid, is connected with the inner surface of this membrane, by 

 means of two horny processes ; this may be regarded as a sort of rudimentary 

 labyrinth. The auditory nerve, which proceeds from the third thoracic gan- 

 glion, forms a swelling as it spreads over the vesicle, which is, by some, re- 

 garded merely as a portion of the nerve. A large tracheal sac, near the audi- 

 tory sac, connected with the third stigma, may perform the oflice of a tym- 

 panum. In certain locusts, the organ of hearing is still more curiously placed, 

 viz. , on the chief segment of the front limbs ; it consists also of a vibrating 

 or tympanic membrane, sometimes superficial, sometimes embedded in a 

 cavity, having a slit-like aperture ; near it is found a tracheal chamber, and 

 the nerve spreads out upon it, in the form of fine parallel striae. In the My- 

 riapoda, organs of hearing have not yet been found. The Arachnida appear 

 to possess considerable auditory sense, but no special organ of hearing has 

 been discovered in them. Amongst the larger Crustacea, the organ of hearing 

 is now said to be usually situated in the basal joint of the first pair of antennae. 

 In this situation, for example, in the lobster and river crawfish, there ex- 

 ists a hollow chamber opening externally by a narrow slit in its thin mem- 

 branous walls, and occupied by a sac filled with water, in which are frequently 

 found minute particles of sand, which have entered from without. On one 

 side of this chamber is a fine striated structure ; a nerve which arises with the 

 nerve of the antennae, from the suboesophageal ganglion, spreads out upon it. 

 A greenish glandular mass, found near this sac, is analogous to the cement 

 gland in the cirrhopods, which are, however, destitute of auditory sacs. The 

 acoustic function of this antennal sac has been doubted, because its small 

 tympanum-like covering membrane seems less adapted to convey vibrations 

 than the firm shell of the animal ; moreover, since in some species it exhibits 

 an opening which permits of the entrance of water into its interior, it has 



