185] LIFE HISTORY OF GORDIUS AND PARAGORDIUS—MA Y 65 



Muscles and parenchyma. The very close relationship between muscle 

 and parenchyma cells is evident from the descriptions of both of the forms 

 studied. In the very earliest stages observed there is no distinction be- 

 tween muscle cells and mesenchyma cells that later make up the paren- 

 chyma. The longitudinal muscle cells can of course from their position be 

 distinguished in a general way from mesenchyme cells; but in many cases 

 cells appear that are partly within and partly without the muscle layer, 

 and on account of the existence of all degrees of intercalation, must be 

 interpreted as belonging to the mesenchyma. Furthermore, the cells that 

 line the hypoderm which later forms the nerve cord are at the earliest stages 

 not distinguishable from the muscle cells lining the rest of the hypoderm. 

 In case of the cloacal muscles all gradations can be found between unmis- 

 takable muscle cells and regular parenchyma cells. All of these muscles 

 except the radiating muscles surrounding the posterior part of the cloaca 

 of the male develop from mesenchyme. The radiating muscles develop 

 from the muscle cells lining the hypoderm that is inturned at the cloaca. 

 From these facts it seems probable that both muscle and parenchyma cells 

 develop from mesenchyme and that the position rather than any inherent 

 properties of the cells determines whether they are to form muscle or par- 

 enchyma cells. 



The fibrils in the longitudinal muscles do not appear until the adult 

 stage has nearly been reached. Normally they arrange themselves in a 

 row completely surrounding the rest of the cell, but in cases of excessive 

 flattening they may appear to be interrupted at the outer edge. Such cells 

 formed the basis of Vejdovsky's contention that the muscles of the Gor- 

 diacea are open toward the hypoderm. 



In the light of the present investigations the descriptions of the peri- 

 toneal linings of epithelial nature must be regarded as resting upon misin- 

 terpretations. Vejdovsky was the most positive advocate of the theory 

 that the parenchyma layers are to be interpreted as true epithelium. 

 Villot (1881, 1887) discovered the true origin of the parenchyma layers, 

 but his interpretation was not universally accepted because it lacked 

 conclusive proof. Von Linstow (1889) also believed that no true epithe- 

 lium was present. Most other workers were not inclined to give any posi- 

 tive statements, except Svabenik, who followed the footsteps of Vejdovsky, 

 and, altho his figures show nothing that contributes in any way to the 

 knowledge of the subject, asserted in the most positive terms the existence 

 not only of true epithelium, primary and secondary body cavities, but 

 also a rudimentary segmentation of the body cavity. 



The present investigations show that in the early stages there are no 

 epithelial layers except the hypoderm and the intestine, that the muscles 

 and parenchyma arise as mesenchyme, and that the mesenteries and 

 peritoneal linings are nothing more than layers of parenchyma. The 



