The fourth subclass, CIRRIPEDIA, comprises the Barnacles and Acorn-shells. These are very unlike any of the other Crustacea, and, in fact, they were long classed by naturalists with the Mollusca. It was not until their larval development was made known that they were recognized as Crustacea. The common Goose Barnacle (_Lepas anatifera_--Plate III.) is found adhering to the bottoms of ships and to floating timber. It has a fleshy stalk or peduncle which is fixed at one end to the supporting object, and bears at the other end a shell, made up of five separate plates, enclosing the body of the animal. The stalk corresponds to the front part of the head, and careful examination may discover at its end, among the hardened cement which fixes it to the support, the remains of the antennules by which the attachment of the young animal was first effected. The body of the animal within the carapace or shell bears the usual mandibles, maxillulæ, and maxillæ, close to the mouth, and six pairs of long, tendril-like feet. These feet have each two branches, composed of numerous short segments and fringed with long hairs. They can be protruded from the slit-like opening of the shell, forming a sort of "casting-net" for the capture of minute floating prey. The Acorn-shells, of which one species (_Balanus balanoides_--Plate III.) is abundant everywhere on our coasts, covering rocks and stones just below high-water mark, differ from _Lepas_ and its allies in having no peduncle. The shell is cemented directly to the rock, and is conical in shape, like a small limpet, with a hole at the top which is closed by four movable valves. [Illustration: _PLATE III_ GROUP OF SPECIMENS OF THE GOOSE-BARNACLE (_Lepas anatifera_), ONE SHOWING THE CIRRI EXTENDED AS IN LIFE. (NATURAL SIZE) (_From Brit. Mus. Guide_) GROUP OF A COMMON SPECIES OF ACORN-SHELL OR ROCK BARNACLE (_Balanus balanoides_)(NATURAL SIZE)] The Stalked Barnacles, like _Lepas_ (suborder _Pedunculata_), and the Sessile Barnacles, or Acorn-shells, like _Balanus_ (suborder _Operculata_), together form the order THORACICA. Of the other orders which compose the subclass Cirripedia, the only one that need be mentioned here is the RHIZOCEPHALA, which comprises strangely degenerate parasites living on other Crustacea. The Cirripedia are unlike nearly all other Crustacea in the fact that, with few exceptions, they are hermaphrodite, having both sexes united in each individual. In certain species of the Stalked Barnacles, however, there are minute male individuals that are attached, like parasites, to the large hermaphrodites. In a few species the large individuals only possess female organs, so that the separation of the sexes is complete. The remarkable larval metamorphoses of Cirripedes and the modifications of structure presented by some parasitic forms will be described in later chapters.