478 Mr. J. E. Gray on the Genus Rhizochilu3. 



if by so doing the animal commits voluntary suicide, or has a 

 prolonged existence ; if the latter, one should expect that it must 

 be of a very torpid or lingering description, as the animal is en- 

 tirely precluded from procuring its usual or indeed any other 

 food for its subsistence, and the supply of water for respiration 

 which can enter by the single siphon must be of a very limited 

 quantity, there being only one aperture for its entrance and exit, 

 in comparison with the continued current which usually circu- 

 lates over the gills when the two apertures (one for entrance and 

 the other for exit), which always exist in all Mollusca, are open 

 for the purpose. 



Many gill-bearing univalve mollusks are stationary ; some, as 

 the Magilli for example, live in holes like the bivalves in mass- 

 ive corals ; and the Vermeti in tubes attached to the surface of 

 marine bodies ; but these animals keep themselves on a level 

 with the surface of these bodies, even when enlarging like corals. 

 The genera Hipponyoc and Calyptra form a free cup-like base be- 

 neath their foot, and the genera Pedicularia, Sabia, Tectura and 

 Patella sink pits into the surface of the shell or coral on which 

 they happen to be attached; but all these animals retain the 

 power, like the bivalve mollusks, of having two apertures for the 

 water to their branchial cavity; but I cannot find in either of the 

 two closed specimens of the genus Rhizochilus in Mr. Cuming's 

 collection any trace of a second aperture. 



Many of the lung-breathing Mollusca cover the mouth of their 

 shell after the animal is withdrawn during the very dry, warm, or 

 cold weather with a membranaceous or calcareous epiphragma, 

 the animal during the time sinking into a torpid condition ; but 

 these animals have the power, at the first recurrence of damp 

 weather, to remove this cover, which is not the case with the hard 

 shelly secretions which cover up the mouth of the shell of Rhi- 

 zochilus. 



Many years ago I observed a somewhat similar phenomenon 

 to that noticed by Dr. Steenstrup in a species of Vermetus, but 

 I did not describe it, hoping to obtain more certain information, 

 as I could not then assure myself that the contraction of the 

 mouth of the tubular shell was the work of the animal which 

 formed it, or of a parasite which adopted it as an habitation after 

 the mollusk was dead. 



These Worm- shells are of a dark brown colour, cancellated on 

 the surface, much contorted together, with a free, erect end 

 and a circular aperture. The mouth of many of these erect 

 tubes are covered over with a thin, convex, shelly arch, with only 

 a small hole in its centre not more than one-tenth part of the 

 size of the mouth of the open tubes (PL XVII. B. figs. 4, 5, 6). 

 They were brought from the African coast of the Mediterranean, 



