'2j& ORGANS OF THE SENSES. 



exterior covering of the head in these air-breathing entomoid 

 classes, and the softness and transparency of the vestibular 

 membrane which receives the sonorous vibrations, render 

 more obvious the position and nature of the organs of 

 hearing in them, than if the whole exterior of the body were 

 covered with a more soft and uniform skin, and it is pro- 

 bably in part from this circumstance that a more simple 

 rudiment of these important organs has not hitherto been 

 observed in the higher forms of helminthoid animals which 

 already present the rudiments of almost all the complex 

 organs of insects. From the thick and solid calcareous 

 covering of the body in the higher Crustacea, their organs of 

 hearing, with their exposed vestibular membrane, are most 

 obvious, especially in the active macrurous decapods, as 

 the lobster and cray-fish, where they present a prominent 

 circular aperture covered with a thin membrane and situate 

 at the bases of the outer pair of antennee. This exterior 

 covering is calcined in some of the brachyurous species, as 

 the maja, and appears to form a loose operculum provided 

 with distinct muscles for its movements. Within this 

 simple vestibular cavity is a white, soft, lengthened sac, 

 or membranous labyrinth, filled with a transparent ento- 

 lymph, and supporting the small fibrils of an acoustic nerve 

 derived from the great supra- oesophageal ganglia. It passes 

 obliquely upwards for a short distance into the first segment 

 of the antennae, and appears to be still destitute of those 

 solid internal lapilli or cretaceous substances which are 

 commonly found in the labyrinths of higher animals. By 

 opening on the lower and not the upper surface of the 

 antennee, the position of its aperture corresponds with the 

 inverted position of nearly all the other organs of these 

 animals. The exterior margins of this vestibular opening, 

 or fenestra ovalis, are often prominent and almost tubular, 

 as in the pagurus, where the covering membrane appears 

 fibrous like the membrana tympani of mammalia. The 

 concealed condition of this organ under a solid calcified 

 covering in the brachyurous species, accords with their more 

 limited powers of swimming, and is analogous to the con- 

 cealed position of the organs of hearing within the cranium 

 of cephalopods and osseous fishes. These organs have a 



