Structure of RMzopod Shells. 301 



portion of the Rhizopoda possesses a shell which is perforated 

 by numerous uniformlj 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 tlic 

 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 exce])tion, monaxonic — the pyloni 

 is placed at one pole of the principal axis. The Rhizopod 

 shells of the perforate type are in general spherically Iiomax- 

 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 ' 1 have proposed tlie name of " Pylom " 

 for the principal orifice of the Rhizopod shell. I have there employed it 

 in the tirst place for the orifices occurring in the Eadiolarian skeleton, 

 especially in order to avoid any confusion with the " osculum " (Hiickel) 

 of the central capsule of the Nassellaria and Phoeodaria (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 re.^pects, see the detailed exposition in my 

 'Radiolarienstudien.' 



