NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA — SNODGRASS 93 



nature of the head cavities might equally suggest that they are purely 

 ontogenetic structures, as claimed by Faussek (1899, 1901), for 

 coelomic cavities in general. 



The association of the antennal coelomic sacs with the antennae 

 and the association of the preantennal sacs with preantennal appen- 

 dicular rudiments suggest that in a primitive stage there may have 



Prst 



cCom 



Fig. 40. — Diagrams illustrating two theories of the fundamental structure of 

 the Articulata. 



A, the theory of radial structure, based on a supposed origin of the articulates 

 from a zoantharian polyp, according to which the coelomic sacs represent radial 

 pouches of the enteron, and the nervous system a circumoral nerve ring, seg- 

 mentation of the body being determined by the enteric pouches. 



B, the theory adopted in this paper, which assumes an origin of the articu- 

 lates from a creeping wormlike ancestor, based on the facts that, though the 

 mouth is subapical, the anus is terminal, and that in embryonic development 

 segmentation precedes the formation of the coelomic sacs, which have no con- 

 nection with the enteron ; the mesoderm, being teloblastic, grows forward, and, 

 in its fullest development, may surround the mouth anteriorly, and thus give 

 rise to a secondary radial symmetry in the prostomial region. 



Bpr, blastopore; cCom, cerebral commissure; CS, coelomic sac; Mth, mouth; 

 NR, nerve ring; Prst, prostomium ; VNC, ventral nerve cord. 



been a pair of appendages in the labral region corresponding with 

 the labral sacs. Some writers have contended that the labrum itself 

 represents a pair of united appendages, but since the labrum is imme- 

 diately preoral, a pair of "labral" appendages in an annelid would arise 

 from the base of the prostomiuin. Perhaps, by a long stretch of the 

 imagination, the labral sacs might better be correlated with a hypo- 

 thetical pair of primitive apical prostomial tentacles (fig. 40 B), 



