GROWTH OF SHELL. 55 



hence increases in diameter, and, when concave, in depth. 

 The compartments grow at their basal margins, where they 

 are in contact with the basis ; hence the shell is added to in 

 height, and, owing to the outward inclination of the com- 

 partments, also, in basal diameter ; but the compartments 

 likewise, in most cases, grow along both lateral margins, 

 that is, on the edges of the radii and alse ; and hence the 

 upper part of the shell, also, increases in diameter. The 

 orifice of the shell, moreover, thus becomes enlarged. In 

 some cases the shell is destitute of radii, only sutures being 

 present, that is, the compartments do not grow laterally; and 

 sometimes, as in the whole genus Pyrgoma, there are not even 

 sutures, the compartments having been fused together : in 

 both these cases, the shell can increase in diameter only at 

 the base; and the orifice, it might have been thought, would 

 necessarily have remained, to the destruction of the animal, 

 of the same minute size, as when first formed after the 

 metamorphosis : this 'certainly would have been the case had 

 not the upper ends of the compartments, surrounding and 

 forming the orifice, been nicely adapted always to yield, in 

 a certain limited degree, to the disintegrating influences to 

 which every shell is exposed, but which most Cirripedes 

 can resist ; and the disintegration of the narrow end of a 

 conical tube, of course increases the diameter of its orifice. 

 In Tubicinella, in which the shell is furnished with narrow 

 radii, and does increase in diameter from top to bottom, the 

 increase is not sufficient in proportion to the continued 

 elongation of the shell ; to compensate for this, the orifice is 

 enlarged at short intervals by the breakage of the upper end 

 of the shell, for which purpose (as explained under the 

 genus) it is evidently constructed. Hence we see that, in 

 certain Cirripedes, decay or disintegration, and breakage, 

 are necessary elements in their growth! It is a remarkable 

 fact, which I cannot explain, that in some species in which 

 the orifice of the shell is usually increased by disintegration, 

 if individuals are so situated that they are not exposed to 

 sufficiently energetic disintegrating influences, as may be 

 inferred from the well-preserved condition of the whole 

 surface of the shell, then the radii become developed, and 

 the orifice is increased in size by the diametric growth of 



