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the substance of the tentacle ; the third, a slender nerve, goes to the 
minute eye-speck on the outer side of the base of the tentacle. The 
wavy disposition of these nerves, especially of the rostral pair, clearly 
indicates a provision for considerable elongation of the parts which 
they supply. 
“Thus the genus Lithedaphus differs from the other known forms 
of the Calyptreide in the smaller development of its locomotive and 
respiratory organs, and in the greater development of the organs for 
the prehension and assimilation of food. 
‘Probably no oyster, cemented to its native rock, is more fettered 
in its movements than this highly developed gastropod, to which, 
however, a voluntary detachment of the foot from the gastric plate 
may be possible. M. Dufo however testifies that the only move- 
ment he was able to recognise in his Calyptrea Roissii was an ele- 
vation of the anterior part of the shell, and a corresponding separa- 
tion of it from the supporting plate beneath. 
“The circumstances under which Mr. Cuming discovered his spe- 
cimens would hardly be consistent with a greater extent of motion. 
The foot, therefore, whose normal functions as an instrument for 
traversing space must be restricted to the early age of the Litheda- 
phus, may well offer diminished proportions when the animal has 
chosen a site for the deposition of its ventral plate and has taken up 
a fixed abode. Muscular action being thenceforward much restricted, 
the necessity for extensive respiration is in the same degree abolished. 
The compensation for this abrogation of the power of moving about 
in quest of food is obviously the great development of the proboscidi- 
form head, which, when outstretched in the living mollusk, must 
appear like some worm moving to and fro from between the valves 
of the shell. The tactile organs of sense are co-extended with the 
prehensile organ ; but the eyes, so useful to the young wandering 
mollusk, have much shrunk in the sedentary aged ; and the complete 
elaboration of whatever nutriment may be introduced into the system 
has been provided for by the long and convoluted alimentary canal. 
“ These facts in the anatomy of the Lithedaphus, and their harmo- 
nious adjustment to its peculiar condition as a sessile gastropod in- 
closed in a bivalve shell, leave scarcely any doubt as to this state, 
strange and anomalous though it may seem, being essential to its 
nature and of original design. 
“For assuming that the secretion of a ventral plate may be ex- 
cited by some accidental position of an individual of a species not 
commonly possessing such plate, it would be an extreme hypothesis 
to attribute to the consequent abrogation of the locomotive power a 
gradual and progressive elongation of the head, during successive 
endeavours on the part of the imprisoned mollusk to attain whatever 
food might come within its reach. 
« And admitting that, the supplies of food being casual and scanty, 
the nutriment would require to be longer retained and more com- 
pletely assimilated, to conclude that the alimentary canal thereupon 
aequired additional convolutions, would be still more hazardous. 
But when we find that, the demands upon the respiratory actions 
