RHIZOPODA. 31 



firmly by a horny cement, and it is in some oases simply membran- 

 ous. In rare cases, the shell may be siliceous. In any case, the 

 shell may exhibit one or other of two very distinct types of structure. 

 In the one type (as in Miliola, fig. 9, 6), the shell-walls are not per- 

 forated with holes, and the pseudopodia are therefore all emitted 

 from the mouth or " oral aperture " of the shell. In the other type 

 (as in Discorhina, fig. 9, c), the shell-walls are perforated with a num- 

 ber of little apertures or " foramina," from which the order derives 

 its name. These foramina are the mouths of tixbes which pierce the 

 walls of the shell, and thus establish a free communication between 

 the interior and exterior. In this way the sarcode which fills the 

 inside of the shell is enabled to reach the outer surface, so .as to form 

 a film, from any part of which the pseudopodia may be given off. 

 The presence or absence of foramina is believed to constitute a true 

 structural distinction, and the Foraminifera may be thereby divided 

 into two gi'eat and natural groups {Perforata and Imperforata). 



According to the form of the shell, also, the Foraminifera may be 

 conveniently, though arbitrarily, divided into two sections. The 

 simplest form of shell is seen in such an example as Lagena (fig. 9, 

 a), where the shell consists of but a single chamber ; and the animal, 

 in fact, is nothing more than a little mass of sarcode, surrounded by 

 a calcareous envelope. Lagena, then, may be taken as the type of 

 what are called the " monothalamous " Foraminifera (Gr. monos, 

 single ; thalamos, a chamber) — that is to say, of those forms in which 

 the animal consists of a single segment, and the shell of a single 

 chamber. All the Foraminifera, without exception, commence life 

 as " simple " or " monothalamoiis " forms, like Lagena, but it is com- 

 paratively seldom that they retain this simplicity throughout life. 

 In the great majority of cases the primitive mass of sarcode under- 

 goes constriction or segmentation, according to a determinate law, 

 the segments thus produced remaining in organic connection. In 

 this way we get ultimately a compound organism, composed of a 

 number of little masses of sarcode, all permanently united to one 

 another, and all enclosed in a common shell. In this way are pro- 

 duced forms like Nodosaria (fig 9, d, e), which may be regarded as 

 a good example of these so-called " compound" or " polythalamous " 

 Foraminifera (Gr. polus, many ; thalamos, a chamber). The exact 

 form of shell which is produced by this process will depend upon 

 the direction in which the segments are given off by the primordial 

 segment. If the segments are daveloped in a line, we get such 

 a form as Nodosaria. If they are given off in a spiral direction, 

 each succeeding segment being a little larger than the one before it, 

 and the coils of the spiral all lying in one plane, then we get such a 

 shell as Discorhina (fig. 9, t). This is one of the commonest forms of 



