which we have spoken before. A little crab is known to live near the coast of Peru (_Fabia Chilensis_, Dana), which exists under somewhat different conditions. He chooses, not a bivalve mollusc, but a sea-urchin (_Euriechinus imbecillus_, Verrill), and lodges in the intestine, near its termination, so as to seize as they pass by all those living creatures which are attracted by the odour. Doubtless, the delicacy of our sense of smell is disgusted by such a mode of seeking food; but this predilection may have a reason with which we are not acquainted. There are a considerable number of other species which live under similar conditions. On the coast of Brazil, my son found two couples of crabs in the tube of a very long annelid, narrow at the ends, and wide in the middle. The tube was too small at the end to allow them to escape. These crustaceans had, no doubt, penetrated thither before they had attained their full size. A crab of the family of the Maidæ conceals itself in the substance of a polypidom very common in the Viti Islands, in company with a gasteropod mollusc, and both of them assume the exact colour of the polypidom. This is a new kind of _mimicry_. This crab is known by the name of _Pisa Styx_, the gasteropod is a _Cypræa_, the polyp is the _Melithea ochracea_. A decapod crustacean, the _Galathea spinirostris_, seeks for a _Comatula_, the colour of which it exactly imitates, and with which it lives on the most friendly terms. The holothuriæ, of which we have already spoken, appear to afford an abode to many animals: independently of the _Fierasfer_, the 21 _Holothuria scabra_ of the Philippine Islands regularly lodges in its interior a couple, and sometimes, though rarely, a greater number of pinnotheres belonging to two distinct species. They choose this domicile at an early period, and must be highly delighted with this obscure abode, since they are seen no more, and when they have once entered never quit this living cavern. This observation is due to Professor Semper, who has made us acquainted with so many curious facts of the China Sea and the Pacific Ocean. In the midst of the slender branches of a coral of the Sandwich Islands, the _Pæcilopora cæspitosa_ of Dana, there lives a little crab (_Hopalocarcinus marsupialis_, Stimpson), which is at last completely enclosed by the vegetation of the coral. It only keeps up sufficient communication with the exterior to enable it to procure food. The coral, however, furnishes it nothing but a resting-place in the midst of its tissues.