In this division are included some of the best-known forms.
The creature, in this case, propels itself by the alternate
contraction and expansion of its disk, thus striking the water, and
driving itself forward. These movements take place at regular
intervals, and serve a double purpose. They not only propel, but at
the same time drive the water over the lower surface of the disk. Here
is situated a complicated net-work of vessels, and the fluids of the
body are thus exposed to the influence of oxygen, and receive the
needed aeration. The stroke of the disk, therefore, is not only a
locomotive, but also a respiratory act. The jelly-fishes of this
section move as they breathe, and breathe as they move. Hence the name
which has been given them--_Pulmonigrades_. We find the same admirable
economy of resources amongst the lower animalcules. The cilia which
propel them secure the aeration of the system.
It is evident that the motive apparatus in this section of the
_Acalephae_ is but a feeble one. It only avails in calm weather.
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