Smithsonian miscellaneous collections . to have itssource in the teloblastic ectoderm (Iwanoff, 1928). The forwardgrowth of the mesoderm bands in the Annelida is arrested at themouth of the larva, and the first pair of coelomic cavities lies justbehind the mouth; as a consequence there remains anterior to themouth an unsegmented region of the trunk, known as the prostomium(fig. I A, Prst). With later development, however, the interior ofthe prostomium may become completely occupied by extensions of the SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 coeloniic cavities of the first postoral somit
Smithsonian miscellaneous collections . to have itssource in the teloblastic ectoderm (Iwanoff, 1928). The forwardgrowth of the mesoderm bands in the Annelida is arrested at themouth of the larva, and the first pair of coelomic cavities lies justbehind the mouth; as a consequence there remains anterior to themouth an unsegmented region of the trunk, known as the prostomium(fig. I A, Prst). With later development, however, the interior ofthe prostomium may become completely occupied by extensions of the 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 inthe Polychaeta include tentacles, palpi, eyes, and nuchal organs, butthe cephalic coelome of the annelids is not known to be divided intocorresponding 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 -2Ant 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, andArthropoda, and the apparent homologies of the postoral somites. A, Annelida: the coelomic sacs of mesoderm bands primarily all postoral, butthe prostomium may be invaded by extensions of the coelomic sacs of firstpostoral segment. B, Onychophora: the coelomic mesoderm bands extend intothe preoral region and here contain antennal and possible preantennal coelomicsacs (Evans, 1902). C, Arthropoda: the coelomic mesoderm bands may unitebefore 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 fivesomi
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