THE S ARC ODIN A 



73 



the plasm extends entirely around the shell, the new chamber almost 

 incloses the older ones as in Nodosarina (/?). In other cases the 

 plasm may extend over one sid only of the old shell, and a curvi- 

 linear axis of growth is the result (Fig. 35, A, B, and 36, C). The 

 spiral thus formed may be flat or coiled around a longitudinal axis as 

 in the mollusc Trochus, giving an involute shell. This type, the most 

 highly differentiated of all of the rhizopod shells, exhibits all grades 

 of complexity (Fig. 37). In the highest forms each new chamber 

 has a complete wall, so that the septa between the adjacent chambers 

 consist of two lamellae, while between the lamellae there is fre- 



Fig. 37. A complex polythalamous shell (schematic) of Oferculina. [CARPENTER.] 

 The shell is represented as cut in different planes to show the distribution of the canals 

 (a', a", a'") ; c, c, c, the outer chambers with double walls (d, d, d), one of which is shown in sec- 

 tion (^-). The chambers communicate by apertures at the inner ends of the septa (e), and by 

 minute pores (f). The outside (b) of the shell is marked by the radial septa. 



quently a space filled with a calcareous deposit or what Carpenter 

 ('62) calls the "intermediate skeleton." This inter-lamellar deposit is 

 traversed by a complicated system of canals, and the deposit itself is 

 frequently carried out into external processes and knobs (Calcarind). 

 In the annular or discoid types a process of budding takes place 

 around the entire circumference instead of at a localized area, and 

 concentric circles of chambers are thus formed (Orbitolites). 



The character of the mouth-openings between adjacent chambers 

 depends upon the nature of the outer coating. If the lime casing is 

 perforated by numerous pores through which pseudopodia can be 

 thrust to collect food, then each chamber is sufficient for itself, and 

 the so-called mouth-opening is small; but if the perforations are 

 absent, the mouth-openings are large and allow a free communication 



