GENERAL PRINCIPLES OF ZOOLOGY. 



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

 entire course runs asymmetrically. 



* 



FIG. 85. Halinmma erinaceus, a radiolarian. , external, i, internal, latticed spheri- 

 cal skeleton ; cfr, central capsule; wk , extra capsular soft parts ; ?, nucleus. 



JT 



Sfi 



FIG. 86. Nausithoe^ an acraspedote medusa (after Lang), seen from the end of the 

 greatly shortened main axis, pr, perradii; tr, interradii; ar, adradii (perradii 

 and interradii mark the four planes of symmetry of the animal); *r, subradii: rf, 

 mantle-lobes; f, tentacles; s/f, sensory organs: g, sexual organs; gff, gastric fila- 

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

 opening. 



we 



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

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



