MOLLUSCA. 9 
have these organs placed on the outside of the 
body, sometimes forming prominent warts or pa- 
pille, disposed in rows, or in tufts, sometimes 
resembling little branching trees, and at others, 
arranged as a number of elegant plumes, set, like 
the petals of an exquisite flower, around a circle, 
In the Tunicata, examples of which may be 
found on our rocky beaches, closely adhering to 
the under surface of stones at low water, and 
looking like shapeless masses of a substance some- 
thing between gristle and jelly,—the breathing 
organ is developed to a very great extent. It 
occupies a capacious chamber in the interior of the 
animal, the two sides of which are studded on their 
inner surfaces with little oval cells, arranged in a 
regular pattern of rows. Each of these cells is 
formed by an oval ring of cilia, which, when in 
full play, present a most beautiful and interesting 
spectacle. The accompanying figure, taken from 
the life, is a magnified representation of a tiny 
creature, not larger than a pin’s head, but as trans- 
parent as the purest crystal. The oblong rings 
conspicuously seen are the ciliary cells of the 
breathing organ; but no figure can convey an ade- 
quate impression of the beauty of the sight, when 
the observer gazes upon forty or more of these 
ovals, all set round their interior with what look 
like the cogs on a watch-wheel, dark and distinct, 
running round and round with an even, moderately 
rapid, ceaseless motion. 
One large tribe of the Gasteropoda comprises 
animals, however, which breathe air, and are ter- 
restrial in their habits. Of these the Slugs and 
Snails of our fields and gardens afford familiar 
examples. The delicately-formed, and often bril- 
