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are all aware that the Bernhard crab often takes possession of an empty shell; also that certain worms, as the Sipunculus, fix themselves in other shells, as the pelican's foot (Aporrhaias Pes-pelicani), or in a Dentalium, and by collecting together a little earth at the entrance, construct for themselves a good habitation; and that a species of annelide often takes possession of the tube of Vermetus cancellatus, and contracts the mouth of the shell so as to form a regular dome, leaving only a small hole the size of the parasite. Some have believed that the parasite induces the Vermetus to do this for it, at the same time causing its own death; and from the similarity of the structure of the shell, of the dome, and that of the tube, there appears to be some ground for this idea, but the fact wants patient examination and verification.

It has not been so generally observed that some Crustacea, Cirripedes, Mollusca and Vermes take advantage of the manner in which corals, sponges, and other radiating animals grow, and the ease with which they are induced to turn out of their usual course, to fix themselves on their surface, so as to cause the animals of these kinds on which they have affixed themselves, to form a chamber for their protection. Indeed, this may be also said to be the case, although done by exactly the contrary process, with some vertebrated animals, as for example the larger Cirripedes, as Coronula diadema, Tubicinella Balsenarum, &c. which affix themselves to the skin of the different species of whales, manatees &c, and then, by the gradual addition of new shelly matter to the lower surface of the valves of their shell, gradually sink themselves as they grow into the thick skin of these animals, until the skin entirely protects them; the result being similar to the attack of the Guinea-worm and other parasites, although these animals first enter by a pierced wound, while the barnacles above referred to only enter by causing the absorption of the surface by pressure.

The parasitic Cirripedes are so abundant on corals, that they have been noticed by many authors, who have written or figured these productions, yet they have not been generally understood. Linnæus