4 SOME ASPECTS OF MORPHOLOGY 



1. Segmentation of the Head 



Phylogenetic studies have led to the conclusion that arthropods 

 were originally descendants of annelids. The reader may be 

 reminded that in the latter animals the only differentiated head 

 is the prostomium, which projects in front of the mouth as an out- 

 growth of the first body-segment. Situated within the prostomium 

 is a pair of nerve ganglia forming the archicerebrum or primitive 

 '■-'-•.vv, •• ■■-•^ brain. Each of the post-oral regions or segments 

 : 'y.' * of a Poly chate worm carries a pair of appendages : 



' ^' *^ sT ' * each segment is innervated by its own nerve 



centre, and its cavity is formed by the coalescence 

 of a pair of embryonic ccelom sacs. It was 

 pointed out by Lankester in 1881, and by 

 Goodrich in 1898, that the arthropod head is 

 ° derivable from that of the annelid by the fusion 

 together of the prostomium along with a definite 

 \:; number of the original body-segments which 



Fig 1 An early ^^^^^^^ ^^ • ^^ other words, it is a syncephalon. It 

 embryo of follows, therefore, that those head-segments 

 hfJ'dTvrsion^nTo ^^^ ^^^^^ appendages which are pre-oral in 

 A, the proto- arthropods, must have secondarily acquired that 

 cephalic or^pn^ position. The arthropod brain is similarly a 

 region, and B syncerebrum, formed by the fusion of the annelid 

 orprimI?y trlTnk archiccrcbrum with the nerve centres of certain 

 region. (From of the segments behind. 



Heymons.) Without following the evolution of the 



syncephalon in different groups of arthropods, it may be mentioned 

 that among insects the embryo becomes differentiated at an early 

 period into a greatly expanded protocephalic or primary head 

 region, and a protocormic or primary trunk region (Fig. 1). The 

 primary head, or protocephalon, is represented by the large 

 cephalic lobes in connection with which the acron and first three 

 head-segments develop. The primary head, as thus constituted, 

 is an embryonic and presumably recapitulatory phase only, and 

 the completed head of an insect is formed by the coalescence of the 

 first three protocormic segments with those of the protocephalon. 



