The habits and instincts of certain Crawfishes are very extraordinary. Thus, the _Astaci_ are migratory, and in their travels are capable of doing much damage to dams and embankments. On the Little Genesee River they have, within a few years, compelled the owner of a dam to rebuild it. The former dam was built after the manner of dykes, _i.e._, with upright posts, supporting sleepers, laid inclining up the stream. On these were laid planks, and the planks were covered with dirt. The _Astacus_ proceeding up the stream would burrow under the planks where they rested on the bottom of the stream, removing bushels of dirt and gravel in the course of a night. They travel over the dam in their migrations, _often climbing posts_ two or three feet high to gain the pond above.[22] We have to add a new and eccentric variety of nature--the Pill-making Crab, which abounds at Labuan, Singapore, and Lahore, and is described in Mr. Collingwood's "Rambles of a Naturalist." When the tide is down, this little creature, if stealthily watched, may be seen creeping up a hole in the sandy shore, taking up rapidly particles of the loose powdery sand in its claws, and depositing them in a groove beneath the thorax. A little ball of sand, about the size of a filbert, is forthwith projected, though whether it passes actually through the mouth is not made clear. Pill after pill is seized with one claw, and laid aside, until the beach is covered with these queer little pellets. This is evidently the creature's mode of extracting particles of food from the sand. Mr. Collingwood also describes, as met with on the shores and waters of the China seas, Glass Crabs, whose flat, transparent, leaf-like bodies seem made of fine plates of mica. The dredge brings up many a rich haul of sponges, corals, and gorgoniæ, of the most splendid colours, certain of the sponges harbouring within their cells minute crabs of a new genus. Between Aden and Galle the sea is of a pinkish colour, owing to the immense accumulation of minute kinds of medusæ, in solid masses of red jelly. Over Fiery Cross Reef, the mirror-like sea reveals, at the depth of sixty or seventy feet, this wealth of natural treasures. "Glorious masses of living coral strew the bottom: immense globular madrepores--vast overhanging mushroom-shaped expansions, complicated ramifications of interweaving branches, mingled with smaller and more delicate species--round, finger-shaped, horn-like and umbrella-form--lie in wondrous confusion. Here and there is a large clam-shell, wedged in between masses of coral, the gaping, zigzag mouth covered with the projecting mantle of the deepest Prussian blue; beds of dark purple, long-spined Echini, and the thick black bodies of sea-cucumbers vary the aspect of the sea bottom."[23] FOOTNOTES: