64 



ZOOLOGY 



SECT. 



surface, a corresponding thickness being, probably, removed by 

 solution from the inner side at the same time. 



The shell presents two leading types of structure apart from 

 the form and arrangement of the chambers : either it is of a 

 porcelain-like texture and provided with a single terminal aperture, 

 (Fig. 35, 4), or the texture is glassy and the whole shell perforated 

 with very minute apertures, through which, as well as through the 

 terminal aperture, pseudopods are protruded (Fig. 35, 2). 



In many cases additional complexity is attained by the develop- 

 ment of what is called the supplemental skeleton (Fig. 36,56, s. sk.). 

 This consists of a deposit of calcium carbonate outside the original 

 shell; it is traversed by a complex system of canals containing pro- 

 toplasm, and is sometimes produced into large spines. Foraminifera 



FIG. 87. Hastigerina murrayi. y>hm. vacuolated protoplasm surrounding shell: psd. 

 pseudopods ; ah. shell ; sp. spines. (After Brady.) 



in which this secondary skeleton occurs are sometimes of consider- 

 able size 2-3 cm. in diameter and of extraordinary complexity. 



Many Foraminifera resemble Difflugia in having a skeleton 

 formed of sand- grains, sponge-spicules, and other foreign bodies 

 cemented together by a secretion from the protoplasm (Fig. 36, 1). 

 Some of these are formed on the imperforate type, having the 

 protoplasm protruded from a single terminal aperture ; others on 

 the perforate type, small pseudopods being protruded between the 

 particles forming the shell. 



In many cases the pseudopods are the only portions of proto- 

 plasm outside the shell, whereas in Gromia, as we saw, the shell 

 is invested with a layer of protoplasm, and is thus in strictness 

 an internal structure. In one of the calcareous forms with 



