NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 95 



derm, and give no evidence of having been drawn forward from 

 behind the mouth ; (4) coelomic sacs of the acronal region, so 

 far as known, are best developed in the higher arthropods, and thus 

 do not appear to be primitive structures; (5) the protocerebral and 

 deutocerebral parts of the brain are always connected by preoral 

 commissures, the only postoral cerebral commissure being that of the 

 cephalized tritocerebral ganglia; (6) the mouth and labrum are in- 

 nervated from the tritocerebral ganglia, which would not likely be 

 the case if several other postoral ganglia preceded the tritocerebral 

 ganglia; (7) paired appendages, sense organs, and primarily discrete 

 nerve centers pertain both to the annelid prostomium and to the 

 arthropod acron ; (8) the first antennae of the arthropods never have 



Lm LmMsd 



Lm 



Storri .£^, 



AntMsd 



Fig. 41. — Development of the procephalic mesoderm in Orthoptera. (A, B 

 from Roonwal, 1937; C from Wiesmann, 1926.) 



A, horizontal section of anterior end of 52-hour embryo of Locusta migratoria 

 Linn, showing cephalic mesoderm extending to labrum anterior to stoniodaeum. 

 B, same of 563-hour embryo, with coelomic cavities in labral mesoderm. C, re- 

 construction of head of embryo of Carausius tnorosus Brunner, lateral view, 

 with developing antennal coelom, and mesoderm extending into clypeolabral 

 region. 



Am, amnion; Ant, antenna; AntCS, antennal coelomic sac; AntMsd, antennal 

 mesoderm ; Br, brain ; Lm, labrum ; LmCS, coelomic sac of labrum ; LmMsd, 

 labral mesoderm ; Msd, mesoderm ; Pre, cephalic lobe ; Stom, stomodaeum. 



the structure or musculature of the following appendages ; in the 

 Crustacea they are never truly biramous. 



A brief review of the facts now known concerning the develop- 

 ment of the procephalic mesoderm and nervous system of the arthro- 

 pods will show that the facts are not inconsistent with the idea that 

 both coelomic sacs and multiple nerve centers may be formed directly 

 in the otherwise unsegmented acronal region, and that the phenomena 

 of embryonic development pertaining to the head are most easily 

 understood if they are taken approximately at their face value for 

 phylogenetic recapitulations. 



The cephalic mesoderm of the arthropods is usually continuous 

 with the mesoderm bands of the anterior somites. In a 52-hour 

 embryo of Locusta, Roonwal (1937) says, "it is seen that a pair of 



