796 MICEOSCOPIC FORMS OF ANIMAL LIFE 



present. 1 The shells of Foraminifera are, for the most part, poly- 

 thalamous, or many-chambered (Plates XVIII and XIX), often so 

 strongly resembling those of Nautilus, Spirula, and other cephalopod 

 molluscs, that it is not surprising that the older naturalists, to whom 

 the structure of these animals was entirely unknown, 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 

 withdrawing 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, however numerous they may be, is not only formed, 

 but continues to be occupied 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 the chambers. From what we know of 

 the semi-fluid condition of the Barcode-body in the reticularian type, 

 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 segments nearest the surface or margin is speedily 

 diffused through the entire mass. The relation between these ' poly- 

 thalamous' forms, therefore, and the monothalamous or single- 

 chambered, of which we have already had an example in Groin if. 

 and of which others will be presently described, is simply that, 

 whereas any buds produced by the latter detach themselves to form 

 separate individuals, those put forth by the former remain in con- 

 tinuity with the parent stock and with each other, so as to form :i 

 ' composite ' animal and a ' polythalamous ' shell. 



According to the plan on which the gemmation takes place will 

 be the configuration of the shelly structure produced by the seg- 

 mented body. Thus, if the bud should be put forth from the 

 aperture of a Lagena (Plate XIX, fig. 12) 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 succes- 

 sionally repeated, a straight rod-like shell would be produced, whose 

 multiple chambers communicate 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 protoplasmic- 

 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 

 happens that each segment is somewhat larger than the preceding 



1 Dr. Scliaudinn (Zeitsclir. f. wiss. Zool. lix. 1895, p. 191) has traced the 

 details of nuclear division in Calcituba polymorpha* 



