On _Sponges_.--_Vulsella_ and _Crenatula_ almost invariably occur in large masses of irregular shape, boring into sponges. They are especially abundant on Porifera from the Red Sea. Corals form a favourite home of many species, amongst which are several forms of _Coralliophila_, _Rhizochilus_, _Leptoconchus_, and _Sistrum_. _Rhizochilus_ is a very singular creature, inhabiting branching corals. When adult, it forms irregular shelly extensions of both the inner and outer lips, which adhere to the shafts of the coral, or to the surface of neighbouring shells; at length the aperture becomes completely closed with the exception of the siphonal tube, which becomes long, and consists of the same shelly material. The common _Magilus_ (Fig. 29), from the Red Sea and Indian Ocean, in the young form is shaped like a small _Buccinum_. As the coral (_Meandrina_) to which it attaches itself grows, the _Magilus_ develops at the mouth a long calcareous tube, the aperture of which keeps pace with the growth of the coral, and prevents the mollusc from being entombed. The animal lives at the free, or outer, end of the tube, and is thus continually shifting its position, while the space it abandons becomes completely closed by a mass of solid calcareous matter. Certain species of _Ovula_ inhabit Gorgonia, assuming the colour, yellow or red, of their host, and, in certain cases, developing, probably for prehensile purposes, a pointed extension of the two extremities of the shell. _Pedicularia_, a form akin to _Cypraea_, but with a more patulous mouth, inhabits the common _Corallium rubrum_ of the Mediterranean, and another species has been noticed by Graeffe[163] on _Melithaea ochracea_ in Fiji.