ARTHUR HESS 5 



filaments characteristic of motile flagella in other animals. These 

 flagella differ slightly from those of other organisms in that they 

 possess a relatively thick membrane surrounding the filaments which 

 frequently becomes separated from the filaments so that its rela- 

 tion to the filaments does not appear as intimate as the relatively 

 thin membrane enclosing flagella elsewhere. 



THE MUSCULAR SYSTEM 



(Figs. 1, 15, 16, 17, 20, 21). The ectodermal muscle layer runs 

 essentially longitudinally, while the endodermal layer is predom- 

 inantly transversely oriented. The muscle filaments contained 

 as a cell organelle in the base of the epithelio-muscular and diges- 

 tive cells run parallel to each other, appear to be essentially of 

 one kind, present no cross striation, and hence can be considered 

 as smooth muscle filaments. The muscle fibers run along the meso- 

 glea. They appear to be anchored to the mesoglea by small cyto- 

 plasmic extensions of the cells containing them (see especially Figs. 

 15 and 16 ) . These extensions are frequently more numerous and 

 robust on the ectodermal side and sometimes muscle filaments 

 extend into these cytoplasmic attachment roots. The ectodermal 

 muscle filaments in the cytoplasmic extensions of the base of one 

 epithelio-muscular cell are very intimately related to the muscle 

 filaments of an adjacent epithelio-muscular cell. The extensions of 

 the cells can dovetail with each other in finger-like extensions or 

 can overlap each other. However, the filaments do not pass from 

 one cell to another. The filaments sometimes appear to insert on 

 the cell membrane and when this happens in adjacent cells, an 

 apparent thickening of the adjacent cell membranes occurs and a 

 desmosome-like effect is produced ( Fig. 17 ) . The digestive cells 

 usually do not undergo such an intimate arrangement and adjacent 

 digestive cells are related to each other by relatively smooth 

 membranes. 



There are points along which the mesoglea appears very thin 

 or interrupted and where the ectoderm and endodermal muscle 

 filaments, or at least the membranes of the cells containing them, 

 are practically in contiguity (Figs. 20, 21). Probably some very 

 thin mesogleal substance intervenes between them since, as men- 



