90 
LIFE, IN ITS LOWER FORMS. 
jecting from the upper surface is the broad, flat sail — a 
soft, transparent membrane, but still strong enough for 
the light boat that bears it. 
“ But if a sail be given to beings such as these, whose 
bodies are almost of the same density as the salt water in 
which they live, and at the same time so soft in their con- 
sistency, somo provision must be made to float the tiny 
ship, and keep it buoyant. A mast is likewise needful, 
and, moreover, ballast must be furnished to secure its 
steady course, and keep it from capsizing. All these are 
furnished, and by means as simple as they are efficient. 
Unlike the other Acakphs, whose body is entirely soft, 
these species form in the substance of their backs a shelly 
plate, so thin as scarcely to bo visible, and yet so porous 
that, being filled with air, it is extremely light, so much 
so as to constitute a float, by means of which the creature 
swims. Placed vertically on the top of this stands up 
another lamina of shell, still thinner than the formei', 
planted in the substance of the sail ; this forms the mast, 
and gives sufficient strength and stiffness to enable the 
thin, filmy sail to stand erect against the wind, which 
otherwise would be impossible. The ballast is obtained 
from other sources ; small shells and stones are seized by 
the appendages upon the lower surface of the body, 
which, from their weight, may serve to trim the little ves- 
sel as it scuds along, climbing the billows as they rise and 
fall, or slowly sailing on the tranquil deep.”* 
* “Nat. Hist, of Animals,” i. 189. 
