96 



ZOOLOGY 



horizontal, and were maintained in this position, it would likely happen that the 

 formerly similar poles of the new vertical axis would become unlike, because 

 subjected to different influences. These poles are known as the dorsal and ventral 

 poles. The poles {right and left) of the other transverse or subordinate axis would 

 remain similar, as they are subjected in the long run to similar conditions. This 

 gives us the condition, found in all the higher, free-moving animals, known as 

 bilateral symmetry. It consists, to recapitulate, of (i) a main axis {antero-posterior 



Fig. 51. 



Fig. 51. Diagram of the cross section of a Ash, showing the bilateral symmetry of the parts: 

 df, dorsoventral axis; r(, right-left axis, a.^., paired appendage; b.c, body cavity; c/i, notochord; 

 (/./., dorsal fin; £, gut; A, heart; A. a., haemal arch; m, muscles; n.a., neural arch; sp. spinal cord; s.c, 

 vertebral column. 



Questions on the figure. — In what respects is the symmetry as shown in this 

 cross section different from that shown in the cross section of sea-anemone? Com- 

 pare carefully, and express your conclusions in terms of the axes and their poles. 

 Find other figures in this book illustrating bilateral symmetry. 



axis) usually horizontal and with dissimilar poles; (2) a transverse axis, usually 

 vertical {dorso-venlral axis) with dissimilar poles; and (3) a transverse axis perpen- 

 dicular to the other two, horizontally placed, with poles alike {right-left axis). 

 Such an animal has only one plane (the sagittal) by which it may be divided into 

 symmetrical halves (Figs. 51, 31, loi). 



