NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS IO5 



figure 45, accepts the embryonic facts more literally. It assumes that 

 the archicephalic nervous system of the arthropods, as that of the 

 annelids, has been built up from groups of ganglionic cells centering 



2Tl(Prnt) ^'^^ ^?9^^ 

 Prst^ 

 Pip (Ant) 



Pmt 



(Pnt) — c= 



iTlNv 

 aoCom / 2pj^^ 



\V^AV::^/'pipNv 



SGng- Mth 

 - IGng- 



VNC 



Msd 



iTlNv 

 2TINV 1 



StC 



An 'T'^^ 



Fig. 45. — Suggestions of homologies between the prostomium of the annelids 

 and the acronal region of the arthropod head. 



A, diagram of the anterior segments of a theoretical "lobopod" annelid, with 

 elemental ganglia corresponding with those of the arthropod cerebrum dis- 

 tributed on a preoral commissural arch of the nerve cords, and the potential 

 number of coelomic sacs of the arthropod acron shown in their possible rela- 

 tion to the prostomial appendages. B, diagram of the fundamental structure of 

 the prostomial nervous system of a polychaete annelid (from Binard and Jeener, 

 1928). C, reconstructed frontal section of dorsal fibrillar mass of brain and 

 nerves arising from it in a sedentary polychaete, Sabellaria spinulosa Leuckart 

 (from Binard and Jeener, 1928). D, analytical diagram of the relation of the 

 coelomic sacs of an arthropod to the central nerve ganglia and the associated 

 appendages. 



acConi, anterior cerebral commissure; Acr, acron (prostomium); An, anus; 

 Aiit, antenna; ApGng, apical ganglion; Br, brain; CS, coelomic sac; cv, ventral 

 fibrillar mass of brain ; E, lateral eye ; iGng, protocerebral ganglion ; 2Gng, 

 preantennal ganglion ; sGng, antennal ganglion ; /, //, first and second somites ; 

 IGng, first somatic (tritocerebral) ganglion; mfd, dorsal fibrillar mass of 

 brain ; Msd, mesoderm ; Mth, mouth ; Pip, palpus ; PlpNv, palpal nerve ; Pnt, 

 postantennal appendage ; Pmt, preantenna ; Prst, prostomium ; rd, dorsal root 

 of stomodaeal connective ; SoeGng, suboesophageal ganglion ; StCon, stomodaeal 

 connective ; Tel, telson ; iTl, first tentacle ; 2TI, second tentacle ; iTlNv, nerve 

 roots of median tentacle ; 2TINV, nerve of second tentacle ; VNC, ventral nerve 

 cord ; ZG, zone of growth ; i, labral coelomic sac ; 2, preantennal coelomic sac ; 

 3, antennal coelomic sac. 



upon a fibrous commissural tract arched forward around the mouth 

 and continuous posteriorly with the ventral nerve cords of the somatic 

 system. The primary cephalic ganglia included a median anterior 



