/O MANUAL OF ZOOLOGY. 



completely divided off from its neighbours, or enclosed by a 

 wall of shell. In these " multilocular " or " polythalamous " 

 Foraminifera, therefore, the shell ultimately conies to consist 

 of a series of chambers, separated by partitions of the test, and 

 filled with sarcode. The partitions, however, or " septa," 

 between the different chambers, are perforated by one or more 

 apertures, through which pass connecting bands, or "stolons," 

 of sarcode; so that the sarcode occupying the different 

 chambers is united into a continuous and organic whole. 

 Each segment may give out its own pseudopodia through per- 

 forations in its investing wall (fig. n, c), or the pseudopodia 

 may be simply emitted from the mouth of the shell by the last 

 segment only (fig. n, b). In any case the direction in which 

 the segments are developed is governed by a determinate law, 

 and differs in different species, the form ultimately assumed by 

 the shell depending wholly upon this. The forms, however, 

 assumed by the shells of Foraminifera are extremely variable, 

 even within the limits of a single species, and it would be 

 impossible to notice even the chief types in this place. There 

 are, however, two or three important variations which may be 

 noticed. If the buds are thrown out from the primitive 

 spherule in a linear series so as to form a shell composed of 

 numerous chambers arranged in a straight line, we get such a 

 type as Nodosaria (fig. n, e). When the new chambers are 

 added in a spiral direction, each being a little larger than the 

 one which preceded it, and the coils of the spiral lying in the 

 same plane, we get such a form as Discorbina (fig. n, c\ or 

 Robulina. These are the so-called " nautiloid " Foramimfera, 

 from the resemblance of the shell, in figure, to that of the 

 Pearly Nautilus. From this resemblance the nautiloid Fora- 

 minifera were originally placed in the same class as the Am- 

 monites (Cephalopoda), but their true position was shown by the 

 examination of their soft parts. In the typical nautiloid shell 

 the convolutions of the spiral all lie in one plane j but in other 

 cases, as in Rotalia, the shell becomes turreted or top-shaped, 

 in consequence of the coils of the spiral passing obliquely 

 round a central axis. 



In a few types of the Foraminifera (e.g., in the Dactyloporidce) the succes- 

 sive chambers of the multilocular test have no direct communication with 

 one another, and simply cohere by their walls. In the majority of the 

 compound shells, the successive chambers are so produced, that the septum 

 between any two of them is formed solely by the anterior wall of the older 

 chamber, which thus constitutes the posterior wall of the newer one (fig. 

 II, e). In the highest types of the compound Foraminifera, however, 

 each segment is provided with its own proper wall of shell, each segment, 

 as it is produced, forming for itself a posterior wall which applies itself to 



