The .1leippidcs are a very small family of mi nute species, which live in burrows in the shells of mollusks and some of the larger cirripeds. The mantle lines the burrow, and is attached at one side by a horny disk. The sexes are separate and the females are much larger than the males, which are generally found living within the mantle cavity of the females, but it is not cer tain that they remain there through life.
The Proteolepadidce are a still smaller family. The species are minute, maggot-like animals, with no limbs, parasitic in other crustaceans. So degenerated is the adult female by its par asitic life, that its body consists only of a sac containing the reproductive organs, attached to the crab by numerous fine root-like processes. On account of these, which have arisen from the head of the young, these animals are also known as Illii,;:oecphala or 'root-headed' animals. The best-known genus is Ram,lino, which has long stood as the extreme example of the degenerating effect of a parasitic life. The members of these last two families of barnacles would never be recognized as cirripeds at all from a study of their anatomy alone. But the study of their embryology and the knowledge of their life his tories have revealed clearly their ancestry and relationship; for in early life the larva. of both
these families show the same essential characters as the larvae of other barnacles, and it is only after they have taken up their abode in their host that the degeneration becomes marked. In Romania, the males never become attached to a host, but remain in the larval condition through out life, except for the development of the repro ductive organs.
Barnacles have a special historical interest to every biologist. from the fact that the only ex tensive systematic work in zoology done by Dar win was done upon this group. The classical volumes entitled 'monographs' of living and fos sil Cirripedia, by Charles Darwin. published by the Ray Society of London in 1851-54, will al ways be invaluable to students of the Crustacea. Darwin spent eight years on this work, and it illustrates admirably his patience, thoroughness of investigation, and clearness of expression. An other important work on barnacles is Part XXVIII. of the Challenger reports; Hoek. Re port on the Cirripedia Collected by H.M.S. Chal lenger (London, 1884).