AMOEBOID PROTOZOA (SARCODINA) 215 



water Heliozoa and the marine Radiolaria, secrete spicules which 

 may be loosely connected, forming an envelop, or cemented to- 

 gether, building up skeletons of most delicate and beautiful designs. 

 Sometimes spicules are developed for temporary purposes as the 

 formation of envelopes during encystment. Calcium carbonate is 

 the chief constituent of the shells of marine Foraminifera but is not 

 an element of importance in the shells of fresh -water Sarcodina. 



In a one-celled animal the vital processes of the body, though not 

 different, except in degree, from those of a metazoon, must neces- 

 sarily be simpler. Here all of the Hfe-forces have their origin and 

 all of the metabolic changes take place within the confines of a 

 single cell. 



The entrance of food Into the body in the Sarcodina is a simple 

 process. No mouth being present, material may, in general, enter 

 at any point on the surface. In naked forms of the Amoeba tj-pe 

 the pseudopodia flow around the particle to be ingested and in this 

 way it is enclosed. The pseudopodia of shell-bearing forms draw 

 in the food through the apertures of the shells where it is engulfed 

 by the protoplasm. 



Most of the Sarcodina are herbivorous, their chief food being 

 unicellular plants, as bacteria, diatoms, algae, etc. The plant cells 

 are usually entirely ingested and the soft parts absorbed, after 

 which the indigestible parts are excreted from the body. However, 

 in case of Vampyrella, the parasite penetrates the cells of algae and 

 absorbs their contents. 



Some Sarcodina are known to be carnivorous, feeding upon 

 closely related species. Penard believes that species of Nebcla 

 may make use of the plates of QuadruleUa, Euglypha, Triucma, etc., 

 upon which they feed, in building up their own shells. 



Digestion in all of the Protozoa is intracellular. After the ma- 

 terial enters the body surrounded by a film of water forming a food 

 vacuole, digestive fluids and enzymes act upon it converting it into 

 suitable elements for the life of the cell. Excretion in the Sarco- 

 dina consists, as elsewhere, in the release of waste products. Waste 

 solids may leave the body at any point of the surface. The 

 process is the reverse of ingestion, often consisting in the mere flow- 

 ing away from the material to be discarded. 



