20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



suppose that it took its inception phylogenetically from an attachment 

 of the primary (ectodermal) longitudinal somatic muscles at con- 

 secutive rings on the body wall, and from the accompanying ingrowth 

 of fibers that formed contractile dissepiments between the myotomes. 

 The ingrowth of the septal muscles cut the parenchymatous meso- 

 dermal bands into segmental blocks. This modification and elaboration 

 of the primitive muscular system, and the consequent segmental 

 division of the mesoderm bands, give at once the essential quality of 

 metamerism, and from it there follows as a necessary result the 

 metamerization of other organs, such as external ectodermal struc- 

 tures, the ventral nerve cords, and all structures of mesodermal origin. 



The coelomic cavities first appear in the annelid embryo or larva 

 as cleavage spaces in the segmental mesoderm blocks. Since the un- 

 segmented Platyhelminthes have nephridial organs, it may be assumed 

 that the primitive annelids possessed simple segmental nephridia in 

 the form of internally closed tubules extending into the haemocoele. 

 The primitive coelomic cavities, therefore, were probably spaces 

 formed in the segmented parenchyma for the accumulation of body 

 fluid charged with excretory products. The inner cells of the paren- 

 chyma now formed epithelial walls about the nephric cavities, which 

 became the coelomic sacs ; the outer cells were converted largely into 

 muscles and connective tissue. The muscle cells gave rise to fibers 

 that reinforced the somatic musculature, and eventually came to 

 be its principal constituents. The definitive musculatvire of modern 

 annelids, therefore, is a composite of fibers derived from the larval 

 ectoderm and of fibers formed from the coelomic mesoblast, but in 

 the Onychophora and the Arthropoda the entire musculature appears 

 to be now a coelomic product. There is no reason necessarily for 

 supposing that the primitive mesodermal muscles were functional 

 elements of the coelomic sacs, for, though in ontogeny the mesoderm 

 usually takes the form of two-layered bands of cells, within which 

 the coelomic cavities are formed, it would seem probable that the 

 primitive mesoderm was a loose parenchymatous tissue. The coelomic 

 sacs are specifically the epithelial walls formed about the nephric 

 cavities ; the surrounding muscles were probably generated from the 

 outer undifferentiated cells of the original parenchyma. 



With the later development of the teloblastic somites, into which 

 the germ cells were distributed from their posterior source of pro- 

 liferation, the reproductive products were discharged into the coelomic 

 sacs of these somites, which thus became gonocoelic as well as nephro- 

 coelic in function. Open nephridia or coelomoducts now connected 

 the coelomic cavities with the exterior and served both as excretory 



