18 



INVERTEBRATE MORPHOLOGY. 



of protoplasm traverse the pseuclopoclia as in the Foramiuifera 

 and carry the food-particles to the body proper. This has a 

 delicate ectoplasm and a central endoplasm which is fre- 

 quently highly vacuolated and contains one or more nuclei 



if 



cv 



ef 



cv 



FIG. 6. Actinosph(erium Eiclihornii (after LEIDY). 

 cv and cv = contractile vacuoles. if= iugested food, 



ef= egested food. ps = pseudopodium. 



and contractile vacuoles. In some forms also a skeleton is 

 developed ; it reaches its most perfect form in the stalked 

 Clathrulina, in which it consists of a delicate fenestrated 

 siliceous sphere. 



3. Order Radiolaria. 



The Radiolaria are exclusively marine and are the most 

 complicated of all the Rhizopods. Their pseudopodia re- 

 semble closely those of the Heliozoa, being slender and pos- 

 sessing an axial support. The body varies in shape somewhat 

 in accordance with the shape of the siliceous shell with which 

 almost all the forms are provided. In those forms in which 

 the shell is simplest, as in Thalassicolla (Fig. 7), where it is in 

 reality absent, the body is spherical and is clearly differen- 

 tiated into two regions, not, however, corresponding to the 

 ectoplasm and endoplasm. of other Rhizopods. The centre of 



