CRUSTACEA. 311 



most delicate exploration; and the smaller but similar organs at- 

 tached to the jaws, and called palpi, may also receive some impressions 

 analogous to those of savour or smell. The Crustacea have no true 

 tongue, but the sensations of the membrane lining the interior of 

 the mouth and the cesophagus may guide them in the selection 

 which they make of objects of food. 



The sense of smell is referred to a shallow excavation in the basal 

 joint of the first or median antenna, which opens by a fissure fringed 

 by fine hairs on the upper surface of the joint : this cavity is lined 

 by a soft membrane, supplied by a branch of the internal antennal 

 nerve. The organ is well described and figured in CCXXXV., 

 pi. 9. and 10., in which memoir the author calls attention to the 

 presence of minute grains of sand ; and these, although they must 

 enter accidentally into the cavity, he suggests may act as otolites, 

 surmising the cavities in the small antennae to be acoustic organs. 



The sense of hearing is referred by most authors to a conical pro- 

 cess containing a cavity with a round orifice closed by a membrane, 

 proceeding from the first joint of the second (external and larger) 

 pair of antennae, in the lobster and other Macroura. Behind the 

 process there is continued from its cavity a large sac filled with a 

 clear liquor : a nerve arising in common with the external antennal 

 nerve is spread upon the delicate walls of the supposed acoustic sac. 

 In most of the Brachyura the membrane is stated by Dr. Edwards to 

 be replaced by a small moveable calcareous disc, which is pierced 

 with a small oval opening, over which there is stretched a thin and 

 elastic membrane. The external opening of the ear is closed by this 

 bony disc. A second small plate is so situated as to regulate the 

 tension of the auditory membrane, whilst the rigid stem of the 

 antennae, in which the whole organ is situated, is well adapted to 

 render the auditory vibrations more distinctly perceptible. These 

 vibrations are conveyed through the medium of a vesicle filled with 

 fluid to a branch of the antennal nerve which expands in the vesicle. 



In both the lobster and crab a tube, filled by a greenish substance, 

 is concealed in the lower portion of the shell, and communicates, or 

 is in close contact with, the membranous sac. This structure, and 

 the absence of otolites, has led Farre to suggest that the organ may 

 be olfactory : but the chief parts of the structure bear a close cor- 

 respondence with an auditive vesicle and a tympanitic membrane. 



With respect to the organ of vision, we find in the class Crustacea 

 a most extensive and interesting series of gradations, leading from 

 the simple sessile median eye-speck to two distinct eyes, provided 

 with all the essential optical apparatus and placed upon moveable 

 peduncles. Ocelli or stemmata are combined with compound eyea 



X 4 



