142 Conchology. 



To a similar mechanism Reaumur ascribes the increase of 

 shells. The external surface of that part of the body of the ani- 

 mal which has extended beyond the limits of the old shell, is fur- 

 nished with a great number of canals, in which circulate the ne- 

 cessary fluids for the nutrition of the animal. A great many 

 small particles of a viscid and earthy matter are mixed with these 

 fluids. Now, as these particles are less fluid than those ofwhich 

 the liquids themselves are composed, they approach the sides of 

 the vessels, which are themselves furnished on that side of the ex- 

 ternal surface of the body of the animal, with a great number of 

 pores, which allow them to escape from the vessels, so that they 

 are deposited on the external surface of these tubes, or rather in 

 that body of the animal itself, which is uncovered by the shell. 



These particles of earthy and viscid matter having reached the. 

 surface of the body of the animal, readily unite with each other, 

 and with the extremity of the old shell, especially when the ex- 

 cess of moisture is dissipated; and thus by their union they com- 

 pose a small solid body, which is the first layer of the new addi- 

 tion. Other particles of similar matter continuing to escape in 

 the same way from the excretory vessels of the animal, form a 

 second layer from the first; afterwards a third, and a fourth, or 

 more till the new part of the shell has acquired sufficient consist- 

 ence and thickness. It is however observed to continue thinner 

 for a certain time than a former opening, till the increase of the 

 animal requires another enlargement of its covering. 



When a testaceous animal is going to enlarge its shell, as for 

 mstance the common snail, the body projects from the opening. 

 It is then seen to attach itself to a wall or some other solid sub- 

 stance, and the portion of its body which is unprotected by the 

 shell, is soon covered with the fluids which are excreted from its 

 surface. The peUicle which they produce when the fluid dries, 

 is at first an elastic, but gradually assumes more consistence, and 

 becomes at last similar to the old part of the shell. If in this 

 stage of the process a bit of the shell is broken and removed, 

 without injuring the body of the snail, the skin of the animal is 

 soon covered with a fluid, which gradually thickens, and becomes 

 solid. Twenty-four hours after the operation, a fine crust may 

 be observed, which constitutes the first and external layer, for re- 

 pairing the breach which was made. At the end of some days 

 this layer has become thicker, and in ten or twelve days, the new 

 piece of shell which is formed, has acquired the same thickness as 

 that which was removed. In making this experiment, certain 

 precautions are necessary, otherwise there is some risk of its fail- 

 ure. If, after the broken piece of the shell has been removed, 



