92 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



stomial coeloni of the annelids is a continuation from the coelomic 

 cavities of the first somite, it is claimed by Binard and Jeener (1928) 

 that the prostomial cavities of the spionid Scolelepis fuliginosa belong 

 to a distinct pair of mesodermal sacs associated with the palpi. In the 

 Onychophora and Arthropoda the mesoderm likewise extends into 

 the head region at the sides of, or before, the stomodaeum (fig. 41 A), 

 and is usually excavated by a pair of well-developed coelomic sacs 

 pertaining to the antennae (C, AntCS) ; but in the arthropods there 

 may be formed also a pair of sacs pertaining to transitory preantennal 

 appendages (fig. 42 B, PrntCS), and even a third pair in the labral 

 region (D, LinCS). The position of the antennal sacs, as that of the 

 antennal rudiments themselves, is somewhat variable in diliferent 

 arthropods, both structures being in some cases postoral, in others 

 adoral, and again preoral ; in the Onychophora the antennal sacs are 

 decidedly preoral, though their posterior mesal ends embrace the 

 stomodaeum and give rise to some of the stomodaeal muscles. The 

 preantennal sacs are usually slightly preoral ; the labral sacs lie directly 

 before the mouth. 



When we consider that the forwardly growing mesoderm bands, in 

 their fullest development, should finally meet in front of the blasto- 

 pore, it is evident that coelomic cavities formed in the cephalic region 

 must assume adoral and preoral positions with their axes centering 

 in the mouth (fig. 40 B). Being thus radial in position, the cephalic 

 coelomic sacs cannot represent "somites" in the manner of the paired 

 sacs lying posterior to the mouth, which are transversely opposed 

 to each other. Hence, the assumption that these anterior sacs 

 represent "cephalized somites" is inconsistent with the anatomical 

 conditions that arise in the acronal region of the trunk. Moreover, 

 as we have seen in a study of the annelids, the coelomic sacs them- 

 selves do not determine metamerism ; the segmentation of the postoral 

 parts of the mesoderm bands is secondary to metamerization of the 

 primary somatic muscular system, and the coelomic cavities are later 

 formed probably for physiological purposes. The coelomic sacs, there- 

 fore, correspond with the somites in the segmented part of the trunk, 

 but similar mesodermal cavities might be formed for the accumulation 

 of waste products in an unsegmented region such as the prostomium. 

 The usual absence of well-difYerentiated coelomic sacs in the annelid 

 prostomium, and the fact that the fullest development of the head 

 sacs is found in the higher arthropods indicate that the formation of 

 cavities in the cephalic mesoderm is a secondary accompaniment of 

 advancing organization in the prostomial lobe ; but the temporary 



