CHAPTER XII. 



SPECIAL MORPHOLOGY AND TAXONOMY OF THE 

 SARCODINA. 



The term Sarcodina was introduced by Biitschli in honor of 

 I )u jardin whose studies on the protoplasm of the Foraminifera led 

 him to believe that the living substance of these forms is simpler 

 than that of other living things and justifying his name for it — 

 sarcode. The peculiarity upon which Dujardin based his con- 

 clusion constitutes the essential difference between these types and 

 other groups of the Protozoa. A definite cell membrane is usually 

 absent and the body protoplasm in general is more fluid and more 

 tenuous than in other types. In the absence of confining mem- 

 branes and with the play of internal forces, protoplasmic processes 

 — pseudopodia are put forth so that the contour of the body 

 may be constantly changing, a phenomenon expressed by the term 

 ameboid movement. 



The great majority of Sarcodina are suspended or floating forms 

 (Heliozoa, Radiolaria) and the ground type is homaxonic or spher- 

 ical, but creeping forms are characteristically flattened, while minor 

 variations of the spherical form lead to the greatest variety of radial 

 ellipsoidal and cylindrical types. They vary in size from a few 

 microns to many millimeters while some forms of fossil Foraminifera 

 are from 1 to 3 inches in diameter. 



Unlike organisms in the three other great groups of Protozoa 

 the cortex of the Sarcodina rarely shows much structural differ- 

 entiation. In the majority of cases it is soft and highly vesicular 

 but shows a marked tendency to form an outer or inner lifeless 

 mantle of chitin. Such lifeless mantles or membranes may be 

 tightly fitting or may be in the nature of tests or houses. They 

 may be of pure chitin as in Coehliopodium, Gromia, etc., or, more 

 frequently, of chitin impregnated with iron oxides, or still more 

 frequently may serve as a substratum on which foreign particles 

 or plates and scales manufactured by the organism are cemented, 

 as in the majority of testate rhizopods. Or between lamellae of 

 chitin precipitation of calcium carbonate leads to the formation of 

 the limestone shells of the Foraminifera. Skeletons of silica or 

 strontium sulphate of varied patterns and often of exquisite design 

 are characteristic of the Radiolaria, while spicules, rods and plates 

 of silica are widely distributed amongst Heliozoa and Radiolaria. 

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