82 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



to an extreme by the foraminifera with their elegant, multi- 

 chambered shells. These structures generally are mineralized and 

 offer serious but surely not insurmountable obstacles to thin 

 sectioning. The development of techniques for studying them 

 should be well worth the effort for the light they may cast on 

 phenomena of secretion. 



A brief glimpse of the intricacies of foraminiferan shell structure 

 is provided in a short report by B. Jahn (1953) of an electron- 

 microscope study of shell fragments from recent and fossil species. 

 The shell is pierced by pores that connect on the inner side to 

 cylindrical tubes marked by regular annulae. These annulae are 

 the sites of delicate, lace-like sieve plates extending across the 

 tube lumen. It was not possible to relate these tubes in the isolated 

 shells to the structure of the living inhabitant. 



Some foraminifers and one group of testate amebae secrete 

 shells that are primarily proteinaceous. A species of the latter 

 group, Gromia oviformis, has very recently been subjected to 

 electron-microscopic study by Hedley and Bertaud (1961). 

 G. oviformis is a large marine species that occupies a spherical shell, 

 about 10 /x in thickness, with an oral opening that is rimmed by 

 a mucopolysaccharide capsule. During feeding and locomotion, 

 a pseudopodial reticulum similar to that of Allogromia (p. 75) 

 extends from the oral opening. Hedley and Bertaud were not 

 able to observe pseudopodial structure in their material and 

 directed their attention to the structure of the shell and enclosed 

 cytoplasm. 



The substance composing the proteinaceous shell is dense and 

 homogeneous or sponge-like. It is regularly penetrated by radial 

 canals containing traces of amorphous material. The oral 

 capsule is composed of tubular fibrils, about 12 m/x in diameter, 

 piled in layers. Within each layer the fibrils run roughly parallel 

 to each other but at approximately right angles to fibrils in the 

 adjacent layers. This arrangement is reminiscent of the alignment 

 of collagen microfibrils in some vertebrate connective tissues, 

 but no chemical similarity is suggested. 



Lining the inner surface of the shell are several delicate lamellae 

 with a remarkable structure. Each is composed of minute 

 cylinders, about 10 m/x in diameter and 20 mtx long, their axes 

 perpendicular to the plane of the sheet. Each cylinder is 



