FORM OF THE BODY 



131 



into the air, two sets of locomotor organs might be necessary, 

 but in general the principle of economy determines that only 

 one set of appendages is developed. For those forms which 

 move on the bottom of the sea or on land the locomotor ap- 



I 



A 



Fig. 62. — Diagram of bilateral symmetry (fish), d-v, Dorso- ventral axis; 

 r-l, right-left axis; ap, appendages; b.c, body cavity; ch., notochord; d.f., dorsal 

 fin; g, intestine; h, heart; h.a., haemal arch; m, muscles; n.a., neural arch; sp, 

 spinal cord; v.c, vertebral column. (From Galloway.) 



pendages will necessarily be constructed with reference to the 

 force of gravity. Since this force operates in one direction 

 only, the appendages have a one-sided relation to the body. 

 There are therefore an upper and a lower side of the body, and 

 these two sides are not alike. Moreover, since light, which 



