Structure of Rhizopod Shells. 303 
lation both with each other and with the formation of the 
pylom. This is easily explained by the fact that all depend 
upon the same physiological cause in the soft body secreting 
the shell. All the peculiarities of the monaxon-pylomatic 
type, including even the formation of the pylom, are to be 
referred to a uniaxial differentiation of the sarcode-body, which 
no longer emits its pseudopodia equally distributed on all 
sides, but for the most part, or even exclusively (imperforate 
forms), from one point, namely through the pylom; next to 
this principal effluent point the flow of sarcode is strongest 
at the opposite pole, and, indeed, sometimes, as in the amphi- 
stome Rhizopoda, it is equally strongly developed at both 
poles. By this orientation of the soft body in the direction of 
a primary axis its formative or secretory activity is no longer 
equally great in all directions, but localized in a corresponding 
manner, so that the two poles of the principal axis are distin- 
guished in the way above indicated by radial appendages of 
various kinds from the more indifferent equatorial parts of 
the shell. 
In a very great number of cases it is proved by observa- 
tion that a strengthened main flow of sarcode takes its course 
through the pylom, quite apart from the imperforate Thalamo- 
phora and Radiolaria, in which, from the very nature of the 
case, the whole of the pseudopodia must pass through the 
pylom as the only aperture present. We may therefore with- 
out hesitation regard such an arrangement as a general rule, 
without requiring direct proof for every pylomatic Rhizopod 
shell. From analogy, ¢. e. supported by the numerous 
actually observed cases and the harmonious intimate relation of 
the different parts of an organism which no one can very well 
doubt, this assumption is justified. 
It might perhaps be objected, however, that the pyloms of 
the Rhizopoda being traversed by a stronger flow of sarcode 
does not prove that the latter is also the cause of the forma- 
tion of the apertures; on the contrary, the opposite causal 
nexus might exist and the sarcode cords principally issue 
there, because a more convenient course is offered to them. 
In answer to this objection it will suffice to indicate simply 
that the soft protoplasmic body is the original thing, and the 
hard structure a secondary secretion from it. The soft body 
forms the shell for itself in accordance with its wants, instead 
of arranging itself to suit the shell; the apertures of the shell 
of course serve for the passage of the pseudopodia outwards, 
the small pores for single ones, the great pylom-aperture for 
a larger number of pseudopodia. 
In a number of pylomatic Rhizopoda the development of 
