632 COMPARATIVE ANATOMY 



from the gonadic sacs of fiatworms. The metamerism which is so charac- 

 teristic of the gonads of amphioxus is lost in vertebrates. 



The attempt has been made to compare the mesodermal bands of 

 annelids and chordates. These in annelids arise from a pair of teloblastic 

 cells located in the posterior end of the body. In amphioxus, which is 

 usually taken as the prototype of chordate development, the anterior 

 portion of the mesoderm arises as an outpocketing of the primary endo- 

 derm. A similarity of origin is, therefore, not easily demonstrated. 

 It is true, however, that the mesoderm of annelids and of amphioxus 

 undergoes a similar total segmentation into a series of metameric divisions. 

 The coelomic cavity of amphioxus is primarily connected with the enteron 

 and is therefore an enterocoele, while that of annelids is not. It must be 

 admitted that the case for the annelid theory is not greatly strengthened 

 by the study of the mesoderm. 



One of the most striking contrasts between annelids and chordates is 

 afforded by the notochord. If chordates are the descendants of annelids, 

 it is necessary to assume either that the notochord is a novelty or that an 

 homologous structure is present in annelids. Some morphologists prefer 

 the first alternative. Others assume that the notochord has developed 

 from the " Faserstrang" of annelids, which is a bundle of connective 

 tissue fibers extending along the ventral nerve cord. Both nerve cord 

 and faserstrang are enclosed in a common connective tissue sheath. 

 Histologically, however, there is little resemblance between faserstrang 

 and notochord, and there is no evidence of similarity in development. 



The annelid theory throws no Hght upon the origin of either the 

 cartilaginous or the bony skeleton of vertebrates. 



The absence of gill-slits in annelids and their presence in chordates 

 has led to considerable speculation on the part of supporters of the 

 annelid hypothesis. The most plausible guess as to their origin has been 

 that gill apertures have arisen from paired diverticula of the pharynx, 

 which reached the skin and became perforated. It has also been sug- 

 gested by Dohrn that nephridia have been converted into gill-slits by 

 the attachment of their inner ends to the pharynx, and subsequent 

 shortening. The elongated tubular ectodermal passages of the gills of 

 myxinoids have been mentioned as evidence in support of this assumption. 

 Since, however, the gills of cyclostomes are wholly endodermal, and since 

 in all vertebrates the development of gills is initiated by endodermal 

 pouches, the suggestion that ectodermal nephridia have been converted 

 into gill-slits seems quite incredible. It is true, however, that the ecto- 

 derm is not wholly passive in the development of gills, and that in fishes 

 the ectoderm participates in the formation of internal and external gills, 

 but this evidence by no means justifies the supposition that nephridia 

 have been converted into gills. 



