[Illustration: FIG. 7.—Adult Sacculina (female).]
Lernæocera again, which in the adult condition is found attached to the gills of fishes, has lost the well-developed legs of its Nauplius childhood and become an elongated worm-like creature (Fig. 8), fitted only to suck in nourishment and carry eggs.
[Illustration: FIG. 8.—Adult female Lernæocera.]
Amongst these Nauplii—all belonging to the great group Crustacea, which includes crabs and shrimps—is one which gives rise to an animal decidedly degenerate, but not precisely parasitic in its habits. This Nauplius is the young of the ship’s Barnacle, a curious stalked body, inclosed in a shell of many pieces (Fig. 9). The egg of the Barnacle gives rise to an actively swimming Nauplius, the history of which is very astonishing. After swimming about for a time the Barnacle’s Nauplius fixes its head against a piece of wood, and takes to a perfectly fixed, immobile state of life (Fig. 10). The upper figures represent the Nauplius stage of animals closely resembling the Barnacle: the lower figures show the transformation of the Nauplius into the young Barnacle. Its organs of touch and of sight atrophy, its legs lose their locomotor function, and are simply used for bringing floating particles to the orifice of the stomach; so that an eminent naturalist has compared one of these animals to a man standing on his head and kicking his food into his mouth.
[Illustration: FIG. 9.—Adult Barnacle or Lepas (one of the Cirrhipedes). Natural size.]
[Illustration: FIG. 10.—Development of Cirrhipedes (Barnacle and Sea-acorn). After Huxley.]
Were it not for the recapitulative phases in the development of the Barnacle, we may doubt whether naturalists would _ever_ have guessed that it was a degenerate Crustacean. It was in fact for a long time regarded as quite remote from them, and placed among the snails and oysters; its true nature was only admitted when the young form was discovered.
Other parasitic organisms, which exhibit extreme degeneration as compared with their free-living relatives, might be cited and figured in profusion, did our limits permit. Very noteworthy are the degenerate Spiders—the mites, leading to still more degenerate forms, the Linguatulæ.