276 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



very elaborately coiled, and attain, as in the fossil 

 Nummulites, to a considerable size (more than four 

 inches in diameter ; or, as in some recent species, to 

 a diameter of two inches). Other Rhizopods build up 

 their skeleton, as do many sponges, from the silica 

 dissolved in sea-water, and others, like a number of 

 sedentary worms, take up sand and other foreign 

 products, and weld them into a consistent skeleton. 



In the Heliozoa there may be a more or less 

 gelatinous investment, which, as in the Rhizopoda, 

 may appear at times only, or be permanent ; or there 

 may be a definite skeleton, which is in no case calca- 

 reous. It is most often formed of silex, and its parts 

 are often disconnected. In rare cases a shell is formed 

 of sand only, or of sand and the tests of diatoms. 



The Radiolaria are remarkable for the pos- 

 session of a so-called " central capsule," which is 

 membranous in structure, and is, like the test of 

 Gromia, perforated at one point only, where there is 

 a comparatively large space, or the membrane is per- 

 forated by several spaces, or a number of pore canals 

 (as in the test of the perforate Foraminifera). In 

 addition to this membranous central capsule, most, 

 though not all, Radiolaria have also a skeleton which 

 may or may not penetrate the central capsule. This 

 skeleton is made up of spicules, which either consist 

 of an organic substance, acanthin, as in the Acantho- 

 jnetriche, or of a siliceous compound. These spicules 

 are primarily arranged in a radiating fashion, and are 

 often connected by secondary spicules with one 

 another, the result being forms of the utmost delicacy, 

 and of great beauty (Fig. 112). 



The great variety of skeletal structure which is 

 seen in the Sarcoclina does not obtain in the Infusoria, 

 many of which are extremely active in movement. In 

 various divisions, however, we find that the cuticle be- 

 comes particularly hard, and the so-called lorica (Fig, 



