40 Traces of Unity in the Appendicular 



eventually formed with these arcs may be regarded as 

 accidental rather than essential, the wing and the leg, in 

 the process of development, becoming connected with 

 the end of that arc which happens to be nearest to it. 

 So at least it may be. And if so then instead of being 

 an exceptional organ the wing may have to be looked 

 upon as having the same relationship to the leg as that 

 which obtains between the gill and the leg in the deca- 

 pod crustacean. Nor is there anything in this supposi- 

 tion which is out of keeping with what has been said 

 already about the compound limbs of the aphrodite and 

 other annulata. For these have been seen to be com- 

 posed of two branches, each of which might be either foot 

 or gill, or both at the same time : so that, after all, the 

 connection of the wing and leg in the insect, and of the 

 gill and leg in the decapod, may only be another illus- 

 tration of that doubling of parts, which, in one of its 

 many forms, is met with in the pieds^a-elytres of the 

 aphrodite. 



Without difficulty, also, it is possible to account for 

 the presence of the prolongations of the stomach, or 

 gastric caeca, which pass from one end of the limb to 

 the other in the legs of the Pycnogonum balnearum an 

 arachnidan parasite living upon whalesand which also 

 penetrate to a greater or less depth in the legs and 

 palps of the spider and its congeners. For in order to 

 this all that is necessary is to suppose that the state of 

 things met with in the ray of the asteria is repeated in 

 these limbs that, in fact, the gastric caeca are only so 

 many evidences of what may be regarded as a rudimen- 

 tary phase of development. 



But it is not expedient to dwell upon these and 

 other peculiarities of the sort in the present place, for 

 the only full explanation is to be found, not in any 



