270 



ESSENTIALS OF ZOOLOGY 



This soupy mass of organisms may cover an area of hundreds of 

 square rods. When stimulated, these animals are luminescent and at 

 night frequently give out an attractive greenish or bluish white light; 

 Uroglena is a fresh-water form which is often found in water supply 

 basins and causes a pungent, oily odor and unpleasant taste in the 

 water. Giardia, Trichomonas, Chilomastix, Retortamonas and En- 

 teronomas are all genera with representatives occurring in the diges- 

 tive-tract of man. 



2. Class Sarcodina (sar k6 di' na, fleshy) or Rhizopoda (ri zop' 6 da, 

 root foot). — A distinctive feature of nearly all species of this class 

 is the capacity to form protoplasmic processes called pseudopodia 

 (false feet) which are temporary structures and can be withdrawn. 

 The animal is able to accomplish locomotion by extending the proto- 

 plasm into these pseudopodia. The representatives of this class 



Dijflugia 



Arcella 



Actinophrys 



Fig. g7. — Group of typical Sarcodina. (From Curtis and Guthrie, Textbook of Gen- 

 eral Zoology, John Wiley and Sons, Inc.) 



include many free-living forms as well as numerous parasitic ones. 

 A number of the representatives of class Sarcodina secrete an ex- 

 ternal shell of lime, silicon, chitin, cellulose, or some bind in sand or 

 other solid substances with one of the secretions. The class is com- 

 monly divided into five orders, (a) Amoehina are irregularly-shaped 

 forms with lobelike pseudopodia. Some of the species are naked, 

 and others are covered by a shell. Amoeba proteus is the free-living 

 naked form which is commonly studied. Endamoeha histolytica is 

 the most common parasitic form. Arcella, which secretes its shell, 

 and Diffliigia, which constructs its shell of sand cemented together by 

 a secretion, are two of the most commonly observed shell-bearing 

 forms, (b) Foraminifera is an order of shelled forms whose pseudo- 

 podia are very slender and reticular. The pseudopodia are extended 

 through small pores in the shell. Only a very few of this group 

 live in fresh water. The vast majority are marine, and Glohigerina 



