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GENERAL PRINCIPLES OF ZOOLOGY. 



the heart to the left; and that the digestive tract throughout its 

 entire course runs asymmetrically. 



Fig. 8.5. — Haliomma erinaceuff, a radiolarian. a, external, i, internal, latticed spheri- 

 cal slieleton; ch, central capsule; u7f, extra capsular soft parts; n, nucleus. 



Fig. fX.—Nausithoii, an acraspedote medusa (after Lani,'), seen from the end of the 

 greatly shortened main axis, pr, perradii; ir, intenadii: or, adradii (perradii 

 and interradii mark the four planes of symmetry of the animal); .vr, subradii: rl, 

 mantle-lobes; (, tentacles; ,s/r, sensory oru'ans; j/. sexual organs; (//', gastric tlla- 

 ments; m, subumbrellar circular muscle; in the centre the'cross-shaped mouth- 

 opening. 



Symmetry. — Now, according to the three dimensions of space, 

 we can pass three axes, perpendicular to each other, through the 



