66 THE MICROSCOPE AND ITS REVELATIONS. 



Foraminifera are, for the most part, polytlialamous or many-chambered 

 (Plate xv.); often so strongly resembling those of Nautilus, Spirula, 

 and other Cephalopod Mollusks, that it is not surprising that the older 

 Naturalists, to whom the structure of these animals was entirely un- 

 known, ranked them under that Class. But independently of the entire 

 difference in the character of the animal bodies by which the two kinds 

 of shells are formed, there is a most important distinction between them 

 in regard to the relation of the animal to the shell. For whilst, in the 

 chambered shells of the Nautilus and other Cephalopods, the animal is 

 a single individual tenanting only the last formed chamber, and with- 

 drawing itself from each chamber in succession, as it adds to this another 

 and larger one, the animal of a nautiloid Foraminifer has a composite 

 body, consisting of a number (sometimes very large) of ' segments/ each 

 repeating the rest, which continues to increase by gemmation or budding 

 from the last-formed segment. And thus each of the chambers, how- 

 ever numerous they may be, is not only formed, but continues to be oc- 

 cupied, by its own segment; which is connected with the segments of 

 earlier and later formation by a continuous ' stolon ' (or creeping stem), 

 that passes through apertures in the septa or partitions dividing tlie 

 chambers. From what we know of the semi-fluid condition of the sar- 

 code-body in the Reticularian type ( 397), there can be little doubt that 

 there is an incessant circulatory change in the actual substance of each 

 segment; so that the material taken-in as food by the segment nearest 

 the surface or margin, is speedily diffused through the entire mass. The 

 relation between these ' polytlialamous ' forms, therefore, and the mono- 

 thalamous or single-chambered, of which we have already had an exam- 

 ple in Gromia ( 397), and of which others will be presently described, 

 is simply that whereas any buds produced by the latter detach them- 

 selves to form separate individuals, those put forth by the former remain 

 in continuity with the parent stock and with each other, so as to form 

 a ' composite' Animal and a ' poly thalamous' Shell. 



457. According to the plan on which the gemmation takes place, will 

 be the configuration of the shelly structure produced by the segmented 

 body. Thus, if the bud should be put forth from the aperture of a La- 

 yena (Plate xv., fig. 9) in the direction of the axis of its body, and a 

 second shell should be formed around this bud in continuity with the 

 first, and this process should be successionally repeated, a straight rod- 

 like shell would be produced (fig. 10), whose multiple chambers commu- 

 nicate with each other by the openings that originally constituted their 

 mouths; the mouth of the last-formed chamber, being the only aperture 

 through which the sarcode-body, thus composed of a number of segments 

 connected by a peduncle or ' stolon ' of the same material, could now 

 project itself or draw-in its food. The successive segments may be all 

 of the same size, or nearly so, in which case the entire rod will approach 

 the cylindrical form, or will resemble a line of beads; but it often hap- 

 pens that each segment is somewhat larger than the preceding (fig. 11), 

 so that the composite shell has a conical form, the apex of the cone 

 being the original segment, and its base the one last formed. The 

 method of growth now described is common to a large number of Fora- 

 minifera, chiefly belonging to the genus Nodosarina; but even in that 

 genus we have every gradation between the rectilineal (fig. 10), and the 

 spiral mode of growth (fig. 11); whilst in the genus Peneroplis (fig. 5) 

 it is not at all uncommon for shells which commence in a spiral to ex- 

 change this in a more advanced stage for the rectilineal. When the 



