GENUS ORBITOLITES PHYSIOLOGY ; GROWTH. 
207 
that their attached surface should ever be invested by sarcode. Moreover, several 
of the spirit-specimens which I have submitted to decalcification, have proved to be 
so closely invested by a covering of vegetation, chiefly composed of Diatomacece, 
Desmidiece, and other minute Algae, that I cannot suppose even the free surface of 
their disk to be ordinarily covered by sarcode*. The analogy of other Rhizopods, 
however, would lead us to suppose that the sarcode projects from the marginal pores 
under the form of pseudopodia, and that it is by the introduction of alimentary parti- 
cles (chiefly minute forms of vegetation) through their means into the mass of sarcode 
from which they are put forth, that the fleshy body pervading the entire disk is 
nourished. For although there is nothing like a digestive cavity in any part of it, or an 
alimentary tube passing from one portion to another, still less any vascular commu- 
nication between the segments, yet as the sarcode forms one soft homogeneous mass 
continuous throughout, the body as a whole will receive the benefit of any incorpora- 
tion of new matter with its substance, in whatever situation this may be made. That 
organic particles small enough to pass through the marginal pores, are thus introduced 
into the chambers of the disk, is proved by the curious fact, that the residuum left 
after the decalcification of large and therefore aged disks, whose animal contents have 
not been preserved, consists almost entirely of an assemblage of remains of minute 
Diatomacece, Desmidiece, &c., which have obviously been retained in the interior of 
their cavities, after the assimilation of the nutriment they were competent to afford. 
35. The sarcode-body of the animal, growing at the expense of the nutriment thus 
appropriated, will gradually, it is probable, project itself through the marginal 
orifices, not merely in filamentous pseudopodia, but in quantity sufficient to form new 
segments on the outside of each pore ; and these segments, extending themselves 
laterally, will come into mutual connexion, and will thus form a complete annulus. 
It may be presumed to be by the calcification of the surface of this beaded ring of 
sarcode, that the formation of the shelly zone is accomplished ; and if the calcifying 
process commence on the segments, and extend from these along the surface of their 
connecting stolons, we can understand why the passages that are left for communi- 
cation with the exterior, should arise from the intermediate divisions of the annular 
canal, instead of from the segments themselves. 
36. The addition of new zones usually takes place with the same regularity in the 
complex as in the simple type of structure ; but departures from this regularity, 
occasioned by a want of completeness of particular zones, are more frequent ; and 
this is perhaps to be accounted for by the larger size of the disk, which will tend to 
produce a less intimate dependence of each part of the animal body upon every 
* I have found such an investment also on several dried specimens ; and until I had detached and examined 
this, I should have supposed from its aspect that it was the desiccated flesh of the animal. I have little doubt 
that the “greenish cuticle” described by Mr. Carter as covering his Operculina ardbica (Ann. of Nat. Hist, 
ser. 2. vol. x. pp. 168, 172) and supposed by MM. d’Archiac and Haime (op. cit. p. 52) to be specially con- 
cerned in the formation of the shell, is of the same nature. 
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