Structure of Rhizopod Shells. 301 
portion of the Rhizopoda possesses a shell which is perforated 
by numerous uniformly distributed pores or by several—at 
any rate more than two—pores, and shows in the majority of 
cases a spherical or polyaxonic fundamental form without any 
clearly marked elongated main axis. Another portion of the 
Rhizopoda shows a distinctly marked, usually elongated, main 
axis of the shell, at one or sometimes at both poles of which 
there is an aperture. ‘This aperture is either the sole opening 
which exists in the shell, or when the wall of the shell is 
perforated it is distinguished from the pores of the shell by 
its greater size and frequently by marginal ornamentation and 
similar differences of various kinds. In accordance with the 
characters just mentioned we may distinguish two kinds of 
structure in Rhizopod shells in general, which may be suitably 
designated the perforate-polyaxonic and the pylomatic *- 
monaxonic form-types. The principal and characteristic 
point in these two types of form is the constitution of the 
shell-apertures, whether uniformly perforated or pylomatic. 
It is only in the second place that the proportions of the pro- 
morphological axes come into consideration; these are in 
most cases dependent upon the nature and distribution of the 
shell-apertures and correlated therewith, as is very natural, 
seeing that the latter on the whole agree with the distribution 
and direction of flow of the sarcode passing outwards. The 
Rhizopods belonging to the pylomatic type are, from the 
nature of the case, without exception, monaxonic—the pylom 
is placed at one pole of the principal axis. The Rhizopod 
shells of the perforate type are in general spherically homax- 
onic or polyaxonic; in many cases indeed even here an abbre- 
viated or elongated principal axis is developed; but this never 
presents a pylom at its poles. 
The more or less uniform perforation, in accordance with 
its indifferent character, exerts no persistent influence of 
importance upon the form of the shell, and there is conse- 
quently nothing further specially to be said upon the perforate 
type. 
* In my ‘ Radiolarienstudien’ I have proposed the name of “ Pylom” 
for the principal orifice of the Rhizopod shell. I have there employed it 
in the first place for the orifices occurring in the Radiolarian skeleton, 
especially in order to avoid any confusion with the “ osculum” (Hickel) 
of the central capsule of the Nassellaria and Pheodaria (Osculosa, 
Hackel). As hitherto no unitary designation exists for the principal 
orifice even of the Thalamophora, it may be desirable to embrace the 
structures in question in the Rhizopoda generally under the term “ Pylom.” 
Upon the comparative morphology of the pyloms and allied structures, 
which is interesting in many respects, see the detailed exposition in my 
‘ Radiolarienstudien.’ 
