FORAMINIFERA. 



[ 330 ] 



FORAMINIFERA. 



microscopic marine animals, contained 

 within calcareous shells, from orifices or 

 pores in which tine retractile processes are 

 emitted, by which locomotion and prehen- 

 sion are performed. 



The shells are sometimes simple, con- 

 sisting of a single cell or chamber (Uiiilo- 

 cular, Monothalamous, or Monostegian), as 

 in Uniloculina, Cornuspira, Ovulites, some 

 Trochammina, Layena, Orbulina, Spirillina', 

 but the cells are usually aggregated into a 

 compound shell (Multilocular or Polytha- 

 lamous). In some they are arranged end 

 to end in a straight row (Stichostegian) , as 

 in some Articulina, some Lituoke, Nodosaria, 

 Frondicularia. In others the single row is 

 rolled into a spiral (Helicostegian, Nauti- 

 loid, Turbinoid, or Fusuline), as in Penero- 

 plis, Lituola, Cristellaria, Polystomella, 670- 

 biyerina, the JRotalina, Nummulites, Fusu- 

 lina. Or the cells are arranged in two 

 alternate rows, spirally coiled (Entornoste- 

 gian), as in Valculma, Bulimina. Some- 

 times the cells form two or three alternate 

 rows, but not spirally coiled (Enalloste- 

 gian), as in Polymorpldna, Uvigerina, Textu- 

 laria ; whilst in others the cells are arranged 

 around an (imaginary) axis, upon two or 

 more opposing faces (Agathistegian), as in 

 Miliola, Chilostometta. There are also dis- 

 coidal shells with alternately concentric cells 

 (Cyclostegian), as Orbitolites and Cyclo- 

 clypeus. Many modifications, with di- 

 morphic and even trimorphic modes of 

 growth, also exist j thus Textularia an- 

 nectens (PI. 23. fig. 62) is helicostegian at 

 first, enallostegian subsequently, and sticho- 

 stegian at last ; whilst Biyenerina and Cla- 

 vulina (PI. 23. figs. 50 & 61) have only the 

 alternate and linear modes of growth ; and 

 Spirolina (PI. 23. fig. 12) is first spiral and 

 then linear. Between the chambers are 

 septa, consisting of either single or double 

 plates, perforated by one or more apertures 

 (whence the name Foraminifera), the mar- 

 gins of which, are sometimes more or less 

 prolonged to form tubes, as shown in figs. 

 31 & 41. This tube is sometimes turned 

 inwards (entosolenian) . As the more re- 

 cently formed chambers are often larger 

 than the others, the shells are often more 

 or less conical or pyramidal. The lines of 

 j unction of the chambers, visible externally, 

 are called the septal lines ; these are some- 

 times sunk, sometimes raised into ridges. 

 Frequently the outer chambers extend 

 laterally beyond the inner, so as to conceal 

 them ; they are then said to be embracing. 



In a few of the Foraminifera the sholls ar<> 

 composed of a number of perfectly distinct 

 cells, each with a separate outer orifice 

 (Dactylopora, PI. 23. fig. 53). 



The plan of growth offers no solid ground 

 for the classification of these organisms ; 

 but the character of their shell-structure 

 serves better; for there are two distinct 

 kinds of shell : one white, opaque, and not 

 traversed with tubules (" porcellaneous" and 

 " imperforate "), such, as the Miliolce ; the 

 other subtranslucent and tubular (" vitre- 

 ous " or " hyaline," and " perforate "), such 

 as Nodosarina, Bulimina, and Nummulites. 

 Shells of each kind, however, are liable to 

 become " arenaceous," by particles of sand 

 or minute organisms being taken up in their 

 structure, as Quinqueloculina, Lituola, Tro- 

 chammina, Valvulina, Textidaria, and 



The surface of the hyaline shells presents 

 a punctate appearance, arising from the 

 presence of very numerous foramina, which 

 are the outer orifices of tubules passing 

 through the walls of the shell. The ar- 

 rangement of these tubules and that of an- 

 other set traversing the walls and the septa, 

 as well as, in fact, the general structure of 

 the shell, may be illustrated by a descrip- 

 tion of the shell of Operculina arabica (PI. 

 24. fig. 23), in which they have been 

 carefully traced by Mr. Carter. Here the 

 outer surface, after the removal of a green- 

 ish epidermic layer, is seen to be covered 

 with large and small papillae the former 

 1-2150", the latter 1-8600" in diameter 

 neither of which are present over the 

 septa or at the margin of the shell. Each 

 of the septa encloses within its walls two 

 calcareous tubes, spaces, or channels, ono 

 on each side the intraseptal channels 

 (tig. 26) ; these are about 1-1900" in dia- 

 meter, and in their course give off two sets 

 of lateral branches, terminating upon the 

 two surfaces of the septum in which they 

 run. The tubes communicate at each end 

 with a network of smaller ones ; one set of 

 which ramifies in the upper, the other in 

 the under wall or margin of each chamber ; 

 these are the marginal plexuses (fig. 24 h) ; 

 and the former terminate upon the outer 

 margin of the shell (g g). The inner wall 

 of the chambers is pierced by innumerable 

 tubules about 1-9COO" in diameter, which 

 pass directly downwards from the small 

 papillae on the outer surface. In a vertical 

 section of the shell, in addition to these 

 minute tubes, seven, eight, or more parallel 



