THE WONDERS OF LIFE 



distinguish several groups of them according to the number 

 of the horizontal transverse axes that cut the vertical main 

 axis in the middle. 



Two totally different divisions of the pyramidal types are the 

 regular and the amphithecta pyramids. In the regular pyra- 

 mids the transverse axes are equal, and the ground-surface (or 

 base) is a regular polygon, as in the three-rayed blooms of the 

 iris and crocus, the four-rayed medusae (A-f, i6, 28, 47, 48, etc.), 

 the five-rayed "regular echinoderms," most of the star-fish, 

 sea-urchins, etc. (A-f, 10, 40, 60), and the six-rayed "regular 

 corals" (A-f, 9, 69). 



The amphithecta (or two-edged) pyramids, a special group of 

 pyramidal types, are characterized by having as their basis a 

 rhombus instead of a regular polygon. We may, therefore, 

 draw two imaginary transverse axes, vertical to each other, 

 through the ground-surface, both equipolar, but of unequal 

 length. One of the two may be called the sagittal axis (with 

 dorsal and ventral pole) , and the other the transverse axis (with 

 right and left pole) ; but the distinction is arbitrary, as the two 

 are equipolar. In this lies the chief difference from the cen- 

 troplane and dorsiventral forms, in which only the lateral axis 

 is equipolar, the sagittal axis being unequipolar. We find the 

 bisected pyramid in a very perfect form in the class of the 

 ctenophora (or comb-medusse, A-f, 27), where it is quite general. 

 The striking typical form of these pelagic cnidaria is sometimes 

 called biradial, sometimes four-rayed and bilateral, and some- 

 times eight-rayed-symmetrical. Closer study shows it to be a 

 rhombus-pyramid. The originally four-rayed type, which it 

 inherited from craspedote medusse, has become bilateral by the 

 development of different organs to the right and left from those 

 before and behind. 



Similar rhombo-pyramidal forms to those of the ctenophora 

 are also found in some of the medusas and siphonophora, many 

 of the corals and other cnidaria, and many flowers. The name 

 "two-edged" which is given to this special type is taken from 

 the ancient two-edged sword. Its chief axis is unequipolar, the 

 handle being at the basic pole and the point at the verticle pole; 

 but the two edges left and right are equal (poles of the lateral 

 axis), and also the two broad surfaces (dorsal and ventral, 

 joined by the sagittal axis). 



III. Centroplanb Types. — The natural middle of the body 

 is a plane, the median or chief plane (planum medianum or 

 sagittale) ; it divides the bilateral body into two symmetrical 

 halves, the right and the left. With this is associated the 



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