[305] The mode of fructification, or _conjunction_, as it is termed, in the Algæ, consists in the adhesion of two cells or frustules together, and their fusion into one; from their united contents a mass of granular substance is produced, that becomes consolidated and forms the spore or fruit, which, when arrived at maturity, is set free by the bursting of the cell. Mr. Thwaites has ascertained that the fructification is similar in the Diatomaceæ. [306] Smithsonian Contributions, vol. ii. p. 34. Thus, whilst referring Closterium, Arthrodesma, Euastrum, Xanthidium, Micrasterium, &c. as vegetables, to the Desmidieæ, Dr. Bailey places, _Actinocyclus_, _Campilodiscus_, _Coscinodiscus_, _Ennotia_, _Navicula_, _Gomphonema_, _Pinnularia_, _Triceratium_, &c. among the Diatomaceæ, as animals. Of the animal nature of the microscopic objects which now require our attention, there is however no question, although the zoological position and affinities of many of the organisms included in this survey are still but imperfectly determined. The animals designated _Foraminifera_,[307] or Rhizopoda,[308] are of a more simple structure than the Polypifera and Echinodermata described in the previous chapters; yet as their relics are for the most part presented to the notice of the geologist as aggregations of shells, forming extensive beds of limestone, it will be convenient to treat of them in this place. [307] Foraminifera, _i.e._ bearing foramina--a name derived from the minute openings in the shells and their septa. [308] Rhizopoda; root-like feet; from the long fibrous processes, or pseudopodia. The fact that the fossil remains of Foraminifera, and of _Mollusca_, alike consist of shells, and constitute strata identical in mineral characters, and deposited under like physical conditions, renders the examination of these Microzoa[309] a fit introduction to the study of the durable remains of the higher order--the Mollusca. [309] Microzoa; from μικρος, _mikros_, small, and ζωον, _zoon_, animal; a convenient term to denote minute animal organisms whose forms can only be defined by the aid of the microscope.