THE METAMERIC APPENDAGES 21 



mentioned order is necessary, however, before it is possible to 

 generalise in this immediate connection. Among Odonata, 

 Tillyard has brought forward evidence indicating that the anterior 

 media (MA) only is retained in this order and it is probable that 

 this same condition will be obtained among Plecoptera. In 

 other orders, excepting the Diptera, it is very doubtful whether 

 MA is ever present. Among Diptera the so-called vena spuria 

 appears to be the vestigial MA, and occurs in the Ptychopterinae 

 and in various members of the Syrphidae. 



Notwithstanding the large amount of study that has been 

 concentrated upon the problem of venational homologies, our 

 conceptions of the subject are not, as yet, by any means stabilised. 

 The results of palaeontological investigations have been to give 

 increased importance to the significance of convexity and concavity 

 of veins as the criterion upon which their homologies largely rest. 

 The fact that the tracheation so often does not precede and, 

 therefore, determine the courses of the veins obviously limits its 

 application. The reader who is desirous of following recent 

 theoretical views should consult papers by Vignon (1929, 1932) 

 and by Vignon and Seguy (1929), whose work has special reference 

 to Diptera. The hypothetical generalised scheme of venation 

 promulgated by Vignon (1929) involves heterodox views which 

 require much wider study before the truth or falsity of his ideas 

 can be determined. 



3. The Metameric Appendages 



The reader may be reminded that the structure of Arthropoda 

 presents a unity of plan rendering it highly improbable that these 

 animals had a polyphylectic origin. Many years ago Lankester 

 claimed that arthropods have developed from a single line of 

 primitive Gnathopoda which arose^ by modification of parapodia- 

 bearing Annelida, and this same view holds to-day. It therefore 

 follows that the annelid parapodia and the arthropod limbs are 

 genetically identical structures. Among the most primitive 

 classes, i.e.^ Crustacea and Trilobita, a biramous type of limb is 

 very general, but in all other Arthropoda morphologically uni- 

 ramous limbs prevail. Evidence of the occurrence of biramous 



