The segments, as the body increases, are agglomerated in six different ways, and these modifications are the basis of M. D'Orbigny's classification. The discoidal forms, as the _Rotalia_, _Rosalina_, _Cristellaria_, &c. are involuted like the nautilus, and divided by septa or partitions, the different lobes of the body occupying contemporaneously every chamber, and being connected by a tube or canal that extends through the entire series. In the spiral forms, the _Textilaria_, &c. the same structure is apparent. These two groups are the most abundant in the cretaceous strata; many beds of the white chalk consist almost wholly of the aggregated shells of the Rosalinæ, Rotaliæ, and Textilariæ.[116] Whatever the form of the body, the filaments always consist of a colourless matter as transparent as glass; they elongate from the base to six times the diameter of the shell. They often divide and subdivide, so as to appear branched. Though alike in form in the different genera, they vary much in their position. In some they form a bundle which issues from a single opening, and is withdrawn into the same by contraction; in others the filaments project only through each of the pores in the shell which covers the last segment; in others they issue from both the large aperture and the foramina. In fine, these filaments or pseudopodia fulfil in the foraminifera the functions of the numerous tentacula in the Asteriadæ, or Star-fishes, serving as instruments of locomotion and attachment. [Footnote 116: See Wonders of Geology, p. 299] Neither organs of nutriment nor of reproduction have been detected. In the genera having one large aperture from which the filaments issue and retract, we can conceive nutriment to be absorbed by that opening; but this cannot be the case in the species which have the last cell closed up; in these the filaments issuing through the foramina are probably also organs of nutrition. M. D'Orbigny considers the Foraminifera as constituting a distinct class in zoology; less complicated than the Echinoderms and the Polypiaria in their internal organization, they have by their filaments the mode of locomotion of the first, and by their free, individual existence--not aggregated and immovably fixed--they are more advanced in the scale of being than the latter. To me they appear to be merely hydra-form polypes of the most simple structure, protected by shells;[117] those composed of different segments, I conceive to be a single aggregated individual, and not a successive series of beings. [Footnote 117: An admirable paper on the "_Polystomella crispa_," by Mr. Williamson, of Manchester, (Trans. Micros. Society of London, vol. ii.) should be consulted on this question.]