Thus the _Ligia_ makes a decided approach to an aërial or terrestrial
mode of life; which, indeed, is strictly the habit of some species with
which every cultivator of flowers is only too familiar. I refer to the
little Woodlice, or Buttons, or Sows, as they are variously called, so
abundant in gardens, especially in pits and frames, and so annoying
for their depredations on our cherished plants. These garden pests are
scarcely to be distinguished as to their structure from the semi-marine
_Ligia_, particularly those flatter and softer species (_Porcellio_),
which do not roll their body into a complete ball.
In all these creatures we find maternal care. The females carry their
eggs in a sort of pouch or sac, placed beneath the breast, which opens
when these are hatched to afford exit for the infant progeny. These
have from the first the same form and appearance as the adult, except
that they have at first six pairs of feet instead of seven. They cling
about the body of the mother, and are carried by her in her rovings
with affecting solicitude.
About the sloping ledges of low-lying rock, especially those which are
covered with young mussels, so closely that between their blue-black
shells, packed and tied together with byssus-threads, you can scarcely
thrust a penknife, we very commonly see slender Worms of a vivid
grass-green hue gliding in and out. Their movements are very graceful,
and if we examine them closely, we discern the presence of a multitude
of minute points along each side of the body, which in turn are thrust
out and as quickly retracted. If we put the little creature into a vase
of water, we see fresh powers and organs of locomotion; for the back
bears a double series of leaf-shaped fins, overlying one another, which
act as broadoars rowing the lithe and graceful worm through the sea.