88 AN INTRODUCTION TO THE STUDY OF FOSSILS 



CLASS A, SARCODINA 



Marine or fresh-water Protozoa with a body w^hich alternately 

 protrudes and retracts first one and then another part into finger- 

 like processes, — the pseudopodia. A contractile v^acuole is 

 present in some fresh water but not in marine forms. Repro- 

 duction is by binary or by multiple (spore-formation) fission. 

 The members of this class are relatively large, many being 

 visible to the naked eye. Some of the fossil forms especially, 

 as Nummulites and Orbitoides, attain giant proportions for 

 Protozoa. 



Derivation of name. — Sarcodina > sarcode > Greek sarx 

 (sark-) flesh. Sarcode was the name applied by Dujardin to 

 what is now called protoplasm. 



The Sarcodina are divided into the two sub-classes, — 



1. Rhizopoda. 



2. Actinopoda. 



Sub-class i, Rhizopoda 



Usually creeping forms with branched pseudopodia (whence 

 the name from Greek rhiza, a root, + pous (pod-) foot). This 

 is subdi\dded into the orders, — 



a. Afncebea. — Amoeboid (changeable) forms, usually with- 

 out shells. Unknown in the fossil state. The genus Amoeba 

 gives the name to the order. 



b. Xenophyophora. — This includes some deep sea forms 

 with a peculiar internal structure and a skeleton composed of 

 foreign bodies, such as sand grains, sponge spicules, etc. (whence 

 the name from Greek xenophyra, foreign bodies, -H pherein, 

 to bear) . Unknown in the fossil state. 



c. Foraminifera. — Skeleton (test) present, composed of cal- 

 cium carbonate, more rarely of sand or of chitin, forming one or 

 more chambers; this is perforated by one or more openings 

 (whence the name from Latin foramen, a hole, + ferrc, to bear). 

 Pseudopodia are long and slender, uniting at intervals. These 

 are typically creeping forms, using their net-like pseudopodia 



