FOKAMINIFEKA. 91 



We have already said that the shells of these minujte zoophytes vary 

 much in form. They are generally many-chambered, each chamber 

 communicating by pores in the walls ; the different gelatinous parts 

 of the animalcules are, in this manner, placed in continual communi- 

 cation with each other. Alcide d'Orbigny, to whom we owe almost 

 all that is known of the class, has distributed them into six families, 

 making the form of the shell the basis of their arrangement. These 

 six families include sixty genera, and more than sixteen hundred 

 species, the families being as follows : 



I. Monostega. Animals consisting of a single segment. Shell of 

 a single chamber. 



II. Stichostega. Animal in segments, arranged in a single line. 

 Shell in chambers, superimposed linearly on a straight or curved axis. 



III. Helicostega. Animal in segments, spirally arranged. Cham- 

 bers piled or superimposed on one axis, forming a spiral erection. In 

 Fig. 21 we have a horizontal section of Faujasina, in which the spiral 

 convolutions are visible on the truncated half of the shell. 



IV. Entomostega. Animal composed of alternating segments form- 

 ing a spiral. Chambers superimposed on two alternating axes, also 

 forming a spiral. 



Y. Enallostega. Animal formed of alternate segments. Non-spiral 

 chambers disposed alternately along two or three axes, also non- 

 spiral. 



VI. Agathistega. Animal formed of segments wound round an 

 axis. Chambers formed round a common axis, each investing half the 

 circumference. 



The simplest form of Foraminifera is illustrated by Fig. 14 (OrbuUna 

 universa), which is a small spherical shell, having a lateral aperture, 

 the interior of which has been occupied by the living jelly, to which 

 the shell owes its existence. In the second order, the shell (Fig. 15), 

 Dentalina communis, advances beyond this simple type by a process 

 of linear budding, the first cell being spherical, with an opening 

 through which a second segment is formed, generally a little larger 

 than the first. This new growth is successively followed by others 

 developed in the same way, until the organism attains its maturity, 

 when it exhibits a series of cells arranged end on end, in a slightly 

 curved line. 



In the next group the gemmation takes a spiral bias, producing the 



