ORGANS OF THE SENSES. 



large vestibular membrane in the palinurus, they appear as 

 small tubercles in the palaemon, and in most of the inferior 

 orders of Crustacea no trace of them can be perceived. 



The slow-moving molluscous animals are less provided 

 with organs for perceiving the properties of outward bodies 

 than the active articulated classes ; but many of the higher 

 pulmonated gasteropods seem both to hear and to smell, 

 although the precise seats of these feelings have not been 

 determined, and the tritonia arborescens emits audible sounds 

 under water, which are, without doubt, intended to be heard 

 by others of the same species, as we see in insects, and 

 probably to serve as a means of communication between 

 these hermaphrodite and almost blind animals, although the 

 organs have not been detected which are appropriated 

 to their perception. The cephalopods which, of all the in- 

 vertebrata, approach the nearest to fishes in their general 

 form, structure and movements, come next to them in the 

 complexriess of their organs of hearing as well as in those 

 of sight. These organs in the cephalopods, as in osseous 

 fishes, are symmetrical and double, placed within the pa- 

 rietes of the cranium, and destitute of external meatus or 

 vestibular membrane. On the sides of the great cephalic 

 cartilage through which the oesophagus passes, and which 

 envelopes the brain, we observe two depressions within the 

 cranial cavity, at its lower part, which are separated by a 

 tough dura mater form the cerebral ganglia. These cavities 

 are separated from each other, and from the exterior mus- 

 cular parts, by the cartilaginous substance of the skull, and 

 they contain each a membraneous sac, or soft labyrinth, 

 which is surrounded by the exterior lymph of Cotunnius, 

 or peri-lymph. The membranous labyrinth on which the 

 acoustic nerves are distributed is filled with a thin ento- 

 lymph, and encloses a solid chalky lapillus, of various forms 

 in different species, composed of carbonate of lime, pre- 

 senting sometimes the appearance of a crystallized rhomboid, 

 and suspended by nervous filaments. Numerous minute 

 blood vessels are seen accompanying the filaments of the 

 acoustic nerves on the parietes of these vestibular sacs. 

 This calcareous substance, like that of the sacculi in the 

 labyrinths of higher animals, serves to communicate more 

 uniformly and distinctly to the accoustic nerves the vibra- 



