l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



protonephridia serve for removal of waste products from the haemo- 

 coele. This statement, however, is not entirely true, for in some of 

 the Polychaeta protonephridia are associated with coelomic sacs, and 

 the primary larval somites of the annelids do not have metanephridia. 

 On the other hand, there can be no question that the coelomic fluid 

 does contain waste products of metabolism. 



A fourth theory, that of Kleinenberg (1886), identifies the primi- 

 tive mesoderm with muscle tissue, and is thus more satisfactory than 

 the other theories because it deals with the beginning of the meso- 

 derm as a functional tissue. Kleinenberg attributes the idea of a 

 muscle origin for the mesoderm to Rabl, who later discarded it, but 

 the theory rests principally on Kleinenberg's studies of the develop- 

 ment of Lopadorhynchus. Kleinenberg claimed that in the larva of 

 Lopadorhynchus the mesoderm is derived directly from the ectoderm 

 at the posterior end of the body, and that the ectodermal m3^oblasts, 

 and the neuroblasts of the ventral nerve cords, arise from a common 

 neuromuscular rudiment. The mesoderm bands, or "muscle plates," 

 become divided into segmental myotomes consequent on metamerism 

 of the body, and the myotomes give rise to the body musculature, 

 including, according to Kleinenberg, the dorsal and ventral longi- 

 tudinal muscles, the parapodial muscles, and the circular muscles of 

 the body wall. Then follows a separation of the muscle plates into 

 parietal and visceral layers in each somite, producing thus the paired 

 coelomic cavities, the peritoneal linings of which are formed by the 

 inner cells of the myotomes. Kleinenberg's theory of the origin of 

 the mesoderm thus gives to metamerism a mechanical significance, 

 since it explains body segmentation as an adaptation to more efficient 

 locomotion. Certainly, when once established, the chief function of 

 metamerism is effective movement of the body, and to this feature 

 the segmented annelids owe their superiority over the unsegmented 

 flatworms. A serious weakness of the muscle theory of the origin 

 of the coelomic mesoblast, however, is found in the fact that so many 

 tissues other than muscle are evolved from it. Muscle fiber is a highly 

 specialized tissue, and it seems hardly likely that epithelial tissue, for 

 example, would be formed from muscle cells, since ordinarily it is 

 epithelial tissue that gives rise to muscle fibers and to the various 

 other specialized tissues of the body. Furthermore, as shown by 

 Meyer (1901), muscle is not formed from the coelomic mesoblast of 

 Lopadorhynchus until after the segmentation of the mesoderm bands 

 and the formation of the coelomic cavities. 



The literature of annelid morphology is replete with discussions 

 on the nature and difference of the "two kinds of mesoblast" ; but 



