-72 THE PROTOZOA 



opening, forms an almost complete investment like a cyst. In many 

 cases this opening is the only means of communication with the sur- 

 rounding medium (Imperforina), but in other cases the entire shell is 

 punctured by minute openings through which pseudopodia pass to 

 the outside (Perforina). These two types of shell are further distin- 

 guished by their appearance ; the Imperforina when seen by reflected 

 light are opaque and like porcelain, while the shells of the Perforina 

 are almost transparent (vitreous). 



Monothalamous or single-shelled Foraminifera may be either im- 

 perforate (e.g- Sqnamnlina, Pilnlina, or Saccammina) or perforate 

 (Lagena). In each group a graded series of shells can be arranged, 

 varying in complexity from the simple monothalamous to the compli- 



B C 



- 36- Polythalamous shell types schematized. [CARPENTER.] 

 A. Linear Nodosaria type. D. Frondicularia form of the Nodosaria type. C. Spiral form 

 of the Nodosaria type. 



cated polythalamous forms (Polystomella, Calcarina). One of the 

 simplest of these shells is that of Cornuspira, where the plasm, as it 

 slowly grows, constantly secretes new shell material and is capable of 

 unlimited extension (Fig. 35, A). It is never divided by septa into 

 separate chambers as in the polythalamous shells. A further step, 

 the simplest of the polythalamous types, is found in shells where the 

 separate chambers adhere end to end as in Nodosaria (C). Here 

 there may be only a slight septum between adjacent chambers, but 

 enough to indicate that growth is periodic, and not constant as in 

 Cornuspira. 



In these chamber-dwelling animals the plasm, as it grows, extends 

 out of the primary shell-opening and reaches to a certain distance 

 down the outside ; new shell material is then secreted, and the process 

 is repeated until a chain of chambers is the result (Fig. 36, A). If 



