RHIZOPODA. 35 



Discorbina, Fig. 4, c) the shell-walls are perforated with a 

 number of little apertures or " foramina," from which the or- 

 der derives its name. These foramina are the mouths of 

 tubes 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 en- 

 abled 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 great groups (Perforata and Im- 

 perforata). 



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

 may be conveniently, though arbitrarily, divided into two 

 great sections. The simplest form of shell is seen in such an 

 example as Lagena (Fig. 4, 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 " monothalamous " forms, like 

 Lagena^ but it is comparatively seldom that they retain this 

 simplicity throughout life. In the great majority of cases the 

 primitive mass of sarcode commences a process of budding, 

 or " gemmation," by which it becomes converted from a "sim- 

 ple " into a " compound " form. The original sarcode-mass, 

 that is to say, begins to throw out buds in some determinate 

 direction; all the buds thus produced remaining connected 

 with one another, and all surrounding themselves with a cal- 

 careous covering. 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. We get then, ultimately, such a form as N~o- 

 dosaria (Fig. 4, d, e), which may be regarded as a good example 

 of these so-called " compound " or " polythalamous " Fora- 

 minifera (Gr. polm, many ; thalamos, a chamber). The exact 

 form of shell which is produced by this process of budding 

 will depend upon the direction in which the buds are given 

 off by the primordial segment. If the buds are given off 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 



