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, some 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 Acalephs, whose body is entirely soft, 

 these species form in the substance of their backs a shelly 

 plate, so thin as scarcely to be 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 former, 

 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. 



