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effective organs of motion, we also find external 
shells, analogous to skeletons; and, lastly, in the in- 
ternal shells of several snails and sepiz, and the ver- 
tebral-shaped cartilages of the cephalopoda, we meet 
with the commencement of the vertebral column or 
true internal skeleton.” 
As in some the rudiment of a bony skeleton is 
barely indicated by cartilage or jelly-like fibre, in 
some it is chiefly or wholly external, in some wholly 
internal; so in plants the substance of the stem is in 
some almost a jelly, as in tremella; in some leathery, 
as in the tulip, the iris, and the rush; in some ex- 
ternally hard, as in the bamboo; in some of extreme 
internal hardness, as in Brazil wood, ebony, box, 
and iron wood. 
Sleep. 
The analogies respecting diurnal and nocturnal 
sleep in different classes of animals and plants, have 
been noticed in the previous remarks on greater or 
less sensibility to light. It is obvious, however, that 
the eye is but a small part of the nervous organism 
affected by sleep. Many animals sleep soundly which 
have no power of closing their eyes, and others 
which have no analogous organ. Rest is as neces- 
sary to plants as to animals: and their especial ra- 
pidity of growth during the night is a proof that the 
organs employed in assimilation had during the day 
been checked in the full discharge of their functions. 
The great propensity to sleep in all young animals, 
when growth is most rapid, bears an obvious analogy 
G 4 
