EARLY DEVELOPMENT 



43 



nected by preoral commissures. The cephalic mesoderm extends forward 

 from the somatic mesoderm bands and, in its fullest development, sur- 



may become excavated by cavities 



rounds the mouth anteriorly; it 

 corresponding with the first an- 

 tennae (ant), the preantennae 

 (pren), and the labrum (Ir)." 



Roonwal (1939), however, is 

 not convinced of the soundness 

 of Snodgrass' interpretation on the 

 ground that not sufficient stress is 

 given to embryological evidence. 

 He concludes that on developmen- 

 tal criteria we must accept the 

 existence of both the labral and 

 preantennary segments in insects 

 and these, together with the five 

 following segments, indicate a 

 seven-segmented nature of the in- 

 sect head. 



APPENDAGES 



It is probable that the early 

 ancestors of the Arthropoda had 

 a pair of appendanges on each of 

 the somites between the prostomi- 

 um and the anus-bearing terminal 

 segment. Although typically seg- 

 mented in postembryonic life, in 

 the embryo these appendages ap- 

 pear as lateral or lateroventral 

 evaginations of the body wall, 

 simple and lobe-like, which only 

 later secondarily may become segmented. The antennae and post- 

 antennae were regarded by most embryologists, at least until 

 recently, as parts homologous with the gnathal appendages and the 

 thoracic legs. Preantennal appendages have also been described in 

 Scolopendra (Heymons, 1901) and Carausius (Wiesmann, 1926). Even 

 the eyes of arthropods were by some believed to be homologous with the 

 appendages, on the basis that the stalks bearing the eyes in Crustacea are 

 segmented and movable and that when an eye is removed under certain 

 conditions an antenna-like structure is regenerated. 



Embryologists in general consider the labrum not as an appendage 

 but as an unpaired lobe of the head that has arisen medially to the neural 



IXi^ 



Fig. 30. — Diagram of relation of coeloinic 

 sacs of an arthropod to the central nerve gan- 

 glia and associated appendages, (acr) Acron. 

 (ant) Antenna. (a7it. 2) Second antenna. 

 (j^oel) Coelomic sac. {com) Commissure. 

 (con) Connective, (ggl) Ganglion, {mes} 

 Mesoderm, {pren} Preantenna. {tel) Tel- 

 son. {From Snodgrass.) 



