THE SHELL. 21 



secrete more than others, so that cavities, filled with fluid, are 

 left in the substance of the shell. 



At maturity the Teredo and Fistulana close the extremity of 

 their calcareous tubes completely, whilst the Aspergillum closes 

 its tube with its characteristic porous or sieve-like disk. The 

 genera of Jonannetinas, near allies of the above remarkable 

 bivalves, have in youth a largely developed foot, but at maturity'' 

 this organ undergoes a retrogressive metamorphosis, becomes 

 atrophied, and is closed in b}'' a calcareous deposit which 

 connects the originally gaping shelly valves into a closed club- 

 shaped tube. Other bivalves, as the symphynote TJnios and 

 Pinna, solder their dorsal line or a portion of it so that continued 

 motion of the valves would be impossible, only that in the first 

 a rupture of one of the soldered wings takes place near its base, 

 so that one of the valves carries both wings whilst the adductors 

 have recovered their power. In Pinna the elasticity of the shell 

 allows some action to the adductor, notwithstanding the sym- 

 phynote hinge-line. Mulleria, which is dimj^ary and locomotive 

 when young, becomes fixed and monomyary when adult, and it 

 solders both the young valves to the beak of the fixed valve 

 whilst the other is free and subject to the action of the adductor. 



A condition similar to the Pholades occurs in the adult 

 Ehizochilus (xliv, 33). Dr. Gray remarks of this singular genus 

 that " the shell, while the animal is growing, is free ; but when 

 the animals have arrived 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 clasping the axis of the coral, Antipathes 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 

 completelj^ surrounded by a solid shell}'- case, having no com- 

 munication 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 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 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 yery limited quantity, 

 there being only one aperture for its entrance and exit. Many 

 of the lung-breathing mollusca cover the mouth of their shell 

 after the animal is withdrawn during very dr}^ weather with a 

 membranaceous or calcareous epiphragm, the animal during 



