If, as seems probable, the body-fluid of these animals is of a lower specific gravity than the sea-water, it will act like the oil-globules of the Copepoda in keeping the animals afloat. Even if the specific gravity be the same, however, the distension of the body with fluid acts in another way, by increasing the surface exposed to friction with the surrounding water, and so retarding sinking. The principle involved is illustrated by the fact that a soap-bubble sinks much more slowly through the air than the drop of water into which it collapses. The same result is produced if the surface is increased by outstanding spines or hairs, just as, for instance, a downy feather sinks slowly through the air, but drops rapidly if it is rolled into a ball between the fingers. This is, no doubt, one function of the spines with which plankton Crustacea, and particularly larvæ, are frequently provided, though they may also serve in some cases as a protection against enemies. The spines have been already alluded to in describing the various larvæ, but it may be noted here that they are most strongly developed in larvæ which live in the open ocean; for example, the most elaborately armed of all Decapod larvæ are the zoëa stages of Sergestes ([Fig. 50]), which, like the adults, belong to the oceanic plankton. The nauplius larvæ of Cirripedes are all more or less spiny, and the spines reach an exaggerated development in the larvæ of the genus Lepas ([Fig. 51]), of which the adults are attached to floating drift-wood or the like, and belong to the oceanic fauna, although hardly to be classed with the plankton.

Fig. 50—The Zoëa Larva of a Species of Sergestes, taken by the "Challenger" Expedition. × 25. (After Spence Bate.)

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Fig. 51—The Nauplius Larva of a Species of Barnacle of the Family Lepadidæ, showing greatly-developed Spines. From a Specimen taken in the Atlantic Ocean, near Madeira. × 11. (After Chun.)

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