SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 95 



coeloniic cavities of the first postoral somite {PMsd), and the pro- 

 stomial walls may thus be lined by a coelomic peritoneum (see Meyer, 

 1901 ; Sokolow, 1911). The external organs of the prostomium in 

 the Polychaeta include tentacles, palpi, eyes, and nuchal organs, but 

 the cephalic coelome of the annelids is not known to be divided into 

 corresponding sacs. 



The coelomic mesoderm bands of the Onychophora also are telo- 

 blastic in their origin. Though they cannot be traced from a single 



-Prn-t 

 -lAnt 



-2 Ant Chi 



;^d~Pdp 



--IM^ _1L_ 



Fig. I. — Diagrams showing the maximum observed development of the coe- 

 lomic mesoderm in the anterior part of the trunk in Annelida, Onychophora, and 

 Arthropoda, and the apparent homologies of the postoral somites. 



A, Annelida : the coelomic sacs of mesoderm bands primarily all postoral, but 

 the prostomium may be invaded by extensions of the coelomic sacs of first 

 postoral segment. B, Onychophora : the coelomic mesoderm bands extend into 

 the preoral region and here contain antennal and possible preantennal coelomic 

 sacs (Evans, 1902). C, Arthropoda: the coelomic mesoderm bands may unite 

 before the mouth and contain antennal, preantennal, and labral coelomic sacs 

 (Carausius, Wiesmann, 1926). 



Ant, coelomic sac of antenna; lAnt, same of first antenna (antennule) ; 2Ant 

 (CM), same of second antenna or chelicera; Bpr, blastopore; Chi (sAnt), 

 coelomic sac of chelicera or second antenna; I-V, coelomic sacs of first five 

 somites; /, coelomic sac of onychophoran jaw somite; L, coelomic sacs of leg 

 somites; Lm, coelomic sac of labral region; Md (Pdp), same of mandible or 

 pedipalp ; Msd, coelomic mesoderm ; Mth, mouth ; iMx, sMx, coelomic sacs of 

 first and second maxillae, or of first and second legs of Chelicerata ; OP, coelomic 

 sac of oral papilla; Pdp (Md), same of pedipalp or mandible; PMsd, prostomial 

 mesoderm ; Prst, prostomium. 



pair of teloblastomeres, they are generated from a multicellular germi- 

 nal area of the blastoderm lying behind the blastopore. In their for- 

 ward growth, however, the mesoderm bands of the Onychophora do 

 not stop at the mouth ; they extend into the head region anterior to the 

 mouth (fig. I B) and are here excavated by a pair of large coelomic 

 cavities {Ant) associated with the antennae, and possibly (Evans, 

 1902) by a pair of small transitory preantennal sacs (Prnt). Though 

 the antennae and their nerve centers in the Onychophora lie well 

 before the mouth, the antennal coelomic sacs are morphologically 



