110 
LIFE, IN ITS LOWER FORMS. 
minute red glands, that are common to the integument 
of the whole class, and this is covered with a series of 
vibratile cilia. 
Conjecture has been busy upon the use of these very 
curious organs in the economy of the animal ; but abso- 
lutely nothing is yet certainly known on the subject. 
Their prehensile power is obvious; but whether this is 
exercised in the way of defensive weapons, or as hands to 
catch food and hand it to the mouth, is among the things 
that we have yet to learn. 
The Sea-urchin may be considered as the type or model 
of the radiate structure, to which we have traced the gra- 
dual approach from the polype-form, through the Feather- 
star, which, in its pentacrine condition, retains the aspect 
of a true Polype. But in nature there is a constant pro- 
gression : and wc must now briefly glance at the transition 
from this perfection of radiism to another sort of symmetry 
— bilateralism, in which there are parts which we can dis- 
tinguish as right and left, dorsal and ventral, head and 
tail. This symmetry now begins to appear, and hence- 
forward is found to characterise the whole range of animate 
existence. 
On turning over stones at the lowest verge exposed by 
the retiring tide, — a means of acquaintance with strange 
creatures which no marine naturalist neglects whenever he 
has the opportunity, — we occasionally find adhering to 
them cylindrical, or rather pentagonal leathery animals, 
very much resembling small cucumbers. They are the 
representatives of an extensive family of this class, called 
Holothuriadm. 
If we place one of these in water we shall see the fol- 
