r 



THE WONDERS OF LIFE 



floating in a liquid, the simplest forms of the radiolaria (actissa), 

 the spherical coenobia of the volvocina and catallacta, and the 

 corresponding pure embryonic form of the hlastula. The 

 smooth sphere is particularly important, because it is the only 

 absolutely regular type, the sole form with a perfectly stable 

 equilibrium, and at the same time the sole organic form which 

 is susceptible of direct physical explanation. Inorganic fluids 

 (drops of quicksilver, water, etc.) similarly assume the purely 

 spherical form, as drops of oil do, for instance, when put in a 

 watery fluid of the same specific weight (as a mixture of alcohol 

 and water). 



The flattened sphere, or facetted sphere {platnosphcBra) , is 

 known as an endospherical polyhedron; that is to say, a many- 

 surfaced body, all the corners of which fall in the surface of a 

 sphere. The axes or the diameters, which are drawn through 

 the angles and the centre, are all unequal, and larger than 

 all other axes (drawn through the facets). These facetted 

 spheres are frequently found in the globular silicious skeletons 

 of many of the radiolaria; the globular central capsule of many 

 spheroidea is enclosed in a concentric gelatine envelope, on the 

 round surface of which we find a net-work of fine silicious threads. 

 The meshes of this net are sometimes regular (generally trian- 

 gular or hexagonal), sometimes irregular; frequently starlike 

 silicious needles rise from the knots of the net- work (A-f, i, 

 51, 91). The pollen bodies in the flower-dust of many flowering 

 plants also often assume the form of facetted spheres. 



II. Centraxonia Types. — The natural middle of the body 

 is a straight line, the principal axis. This large group of funda- 

 mental forms consists of two classes, according as each axis is 

 the sole fixed ideal axis of the body, or other fixed transverse 

 axes may also be distinguished, cutting the first at right angles. 

 We call the former uniaxial {monaxonia) , and the latter trans- 

 verse-axial {staiiraxonia) . The horizontal section (vertically 

 to the chief axis) is round in the uniaxials and polygonal in the 

 transverse-axial. 



In the monaxonia the form is determined by a single fixed 

 axis, the principal axis; the two poles may be either equal 

 {isopold) or unequal {allopola). To the isopola belong the 

 familiar simple forms which are distinguished in geometry as 

 spheroids, biconvex, ellipsoids, double cones, cylinders, etc. A 

 horizontal section, passing through the middle of the vertical 

 chief axis, divides the body into two corresponding halves. 

 On the other hand, many of the parts are unequal in size and 

 shape in the allopola. The upper pole or vertex differs from 



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