34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



ment in favor of the homology of the polychaete palpi with the 

 tentaclelike antennae of the Onychophora ; but evidently it does not 

 prove their further contention (1929) that the palpi are appendages 

 of a secondarily "cephalized" somite, since it must first be demon- 

 strated that coelomic cavities may not pertain to the preoral mesoderm 

 itself. 



THE BODY AND ITS APPENDAGES 



The body of the annelid is the segmented part of the trunk posterior 

 to the acronal prostomium, including the region of the true somites, 

 the zone of growth, and the pygidium ; but the term soma, in a 

 restricted sense, would apply literally only to the region of the somites 

 between the prostomium and the zone of growth or the pygidium. 

 In the Polychaeta the first two somites are generally united with each 

 other in a double segment known as the pcristomiuni (fig. 14, Pcrst), 

 the tentaclelike cirri of which {Cirl, Cirll) take an anterior position 

 closely associated with the prostomium. The "cephalization" of the 

 anterior segments in the polychaetes, therefore, contrasts with that 

 in the arthropods, since, with the latter, the first stage of cephalization 

 is a union of the first somite with the prostomium. In the oligochaetes, 

 however, the first somite and the prostomium may unite to form a 

 composite head as in the arthropods. 



The fundamental demarcation of the annelid somites is the attach- 

 ment of the longitudinal muscle fibers of the body wall and the 

 muscles of the dissepiments on transverse circular grooves of the 

 integument ; but the coelomic sacs when present are strictly intra- 

 segmental, and most of the ectodermal and mesodermal organs are 

 segmentally repeated. The locomotor mechanism of the annelids 

 consists primarily of the somatic musculature and the regulating 

 nerve ganglia, which give movement to the body wall, but it usually 

 includes external adjuncts in the form of bristles or chaetae, and, 

 in the Polychaeta, lobelike segmental appendages, the parapodia. The 

 annelid body musculature should be the basis of the derived arthropod 

 musculature, but there is reason to doubt that the polychaete para- 

 podia are prototypes of the arthropod legs. 



The somatic musculature of the annelids includes the muscles of 

 the body wall, the muscles of the chaetal sacs, and the muscles of 

 the parapodia. The muscle fibers, with possibly rare exceptions, are 

 of the nonstriated type. The musculature of the body wall is of a 

 very simple pattern, so far as the arrangement of the fibers is con- 

 cerned, but it may attain a strong development in the rapacious 

 polychaetes and the burrowing oligochaetes. The longitudinal muscles 



