Among the Philippine Islands, also, a brachyurous crustacean lives in the branchial cavity of one of the _Haliotidæ_, and another on the body of a holothuria. On the coasts of Brazil, F. Müller, during his abode at Desterro, saw some _Porcellanæ_ inhabiting star-fish, not as parasites, as had been supposed, but as true messmates. A crustacean possessed of but little generosity is the _Lithoscaptus_ of Mons. Milne-Edwards. Provided with beak and claws for the purpose of attack, it instals itself, sad to say, in the pantry of a medusa, and instead of making use of its own weapons, takes advantage of the perfidious nematocysts of its acolyte, in order to live quietly at his expense. Under the name of _Asellus medusæ_, Sir J. G. Dalyell has made 22 us acquainted with another messmate of the medusæ which greatly resembles an _Idothea_. Another kind of commensalism is that of the Dromiæ. These crabs are of the ordinary size, and lodge, from their earliest youth, under a growing family of polyps, which increases with them. This colony has for its principal foundation a living Alcyonium, which covers the carapace, and as it develops, adapts itself perfectly to all the inequalities of the cephalothorax; one might consider it an integral part of the crab. Sertulariæ, Corynes, Algæ, develop themselves on this Alcyonium, and the Dromia, masked by this living rock which it carries on its shoulders like the fabled Atlas, marches gravely in pursuit of her prey. She has no fear of arousing the attention of her enemies. The greatest vigilance cannot prevent the sudden attack of these dangerous neighbours. There is in the Mediterranean a species which sometimes comes to our coast. They are also known in the Indian Seas and in the Northern Pacific. Rumphius named the dromia _Cancer lanosus_; it is, said he, a crab which carries grass or moss on its back. It is also mentioned by Renard. Dana has observed a sea-anemone covering a crab in the same manner as the Alcyonium does the dromia, and which is not less dangerous. The mode of life of this anemone has procured for it the name of _Cancrisocia expansa_. In the north of California, a crab (_Cryptolithoides typicus_) covers itself in the same manner with a living cloak which hides it from view, and under cover of which it surprises those whom it attacks. It has already cleared the ground of its prey before any alarm has been given to the neighbourhood.