8 SOME ASPECTS OF MORPHOLOGY 



Apterygota, besides in some of the Orthoptera, it might be claimed 

 with some justification that this condition is primitive. In those 

 examples where it is initiated in the form of paired lobes it might 

 be assumed that a secondary division occurs during development 

 and that the unpaired condition is re-acquired later. Without 

 pursuing this matter further it needs to be taken into account 

 that the process of the amalgamation of the cephalic segments is 

 phylogenetically an extremely ancient phase in arthropod 

 evolution : secondary changes have almost certainly supervened 

 to the extent of partially obliterating the original condition. 

 Furthermore, since the complete arthropod head is believed to 

 have been formed by the successive incorporation of additional 

 postcephalic segments with the primitive cephalon, the most 

 anterior segments are the most ancient in the process of cephalisa- 

 tion. It is these anterior segments, therefore, that are most likely 

 to have become so transformed, in the lengthy process of evolution, 

 as to betray but little evidence of their individuality. If what is 

 known of the segmental constitution of the head in all classes of 

 Arthropoda be taken into account, it appears unlikely that two 

 pre-antennary segments (protocerebral and labral) should occur 

 in the Insect a and in no other class. The subject is discussed 

 by Manton (1928), who doubts whether the labral coelom sacs are 

 independent of those of the protocerebral segment. If this 

 conclusion be correct it may be inferred that only a single pre- 

 antennary segment is involved and that it has undergone secondary 

 division. Further discussion of this problem, however, is scarcely 

 profitable until more evidence is available, andManton's hypothesis 

 is to be regarded as a tentative effort to reconcile conflicting 

 evidence. 



Up to this point the embryological interpretation of the 

 constitution of the insect head has been considered. Hanstrom 

 (1937-30), from a comparative study of the structure of the 

 cephalic ganglia of Annelida and Arthropoda, arrived at a 

 different conclusion. While his theoretical deductions have 

 not gained wide acceptance, his work, as a whole, forms a 

 valuable contribution to invertebrate neurology. The arthropod 

 protocerebrum and deutocerebrum, he claims, are secondary 



