HHizocnTLrs. 205 



<ienus KHIZOCHILUS, Steensti up. 



Dr. Gray remarks of this singular genus that " the shell, while 

 the animal is growing, is free ; but when the animals have arriA^ed 

 at their full development, two or more congregate together in 

 groups, each animal forming a more or less irregular, opaque, 

 white, solid shelly extension of the outer and inner lip clasi)ing 

 the axis of the coral, Anti2:>athes ericoides, or the neighboring 

 shells, or both, and at length entirely closing the mouth of the 

 shell, and firmly attaching the shells to the coral, or to one 

 another, in such a manner that the animal is completely sur- 

 rounded by a solid shelly case, having no communication with 

 the outer world but through the case of the anterior siphon of 

 the mantle, which, by the contraction of the mouth of the shell, 

 has been converted into a shelly tube. This self-immurement of 

 the animal within its shell has not been described in any other 

 mollusk, and one is led to inquire if by so doing the animal com- 

 mits voluntarj' 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 entirely 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 ver}^ limited quantity, there being onl}- one 

 aperture for its entrance and exit. Many of the lung-breathing 

 mollusca cover the mouth of their shell after the animal is with- 

 drawn during very dry weather with a membianac^eous or calcare- 

 ous epiphragm, the animal during the time sinking into a torpid 

 condition ; but these animals have the power, at the first recur- 

 rence of damp weather, to remove this cover, which is not the 

 case with the hard shelly secretions Avhich cover up the mouth of 

 the shell of Rhizochilu^.''''^ 



Notwithstanding the decided opinion given l»y Dr. Gray that 

 the self-immurement of the Rhizochilus is permanent, 1 cannot 

 help thinking that it only continues during a period of hiberna- 

 tion, and as man^^ mollusks have the power of absorbing away 

 partitions in tlieir shells, as well as parts of the columella and the 

 interior thickening of the outer lip, it appears to me that his 

 argument that the hardness and thickness of the pi'ison-walls 



* Ann. Mag. Nat. Hist., 2d Ser., VII, 477, 1851. 



