1916] Malpighian Vessels of Ilaltica Bimarginata 395 



layer). All of the intestinal muscles, both circular and longi- 

 tudinal, are clearly striated. The circular muscles are strongly 

 developed in all but the posterior portion of the colon; there is 

 a single row of them forming a continuous layer around the 

 alimentary canal. The longitudinal muscle layer consists 

 of six large fibres, which are rather regularly disposed, outside 

 of the circular muscles. 



Each vessel trunk lies between two of these longitudinal 

 muscle fibres. As soon as the vessels have become appressed 

 to the wall of the colon, their peritoneal sheaths grow out and 

 join, so that a continuous peritoneal coat is formed, com- 

 pletely surrounding the colon, and enclosing the six muscle 

 fibres and the two Malpighian trunks. As soon as this tunic 

 is complete, each of the trunks redivides into three vessels, 

 which almost immediately begin to "migrate" outside the 

 muscle fibres so as to lie alternately with them. This "slip- 

 ping" or "migration" extends through about twenty sections 

 of six micra each before it is completed. The condition which 

 results characterizes the greater part of the colon and in a 

 typical cross section, one will find lying without the circular 

 muscles, a layer composed of the six longitudinal muscle fibres 

 alternating with the six Malpighian vessels, the whole sur- 

 rounded by a nucleated peritoneum, probably of connective 

 tissue, which, as it represents the investing sheath of the vessels, 

 is a double layer. The two sheets separate at the vessels, one 

 layer passing inside and the other outside of the tubes, but 

 both sheets pass outside of the longitudinal muscles. 



Figure 2 shows the Malpighian vessels after they have 

 separated from the trunk "migrating" outside the fibres of 

 the longitudinal muscles. Figure 3 shows diagrammatically 

 a typical cross section of the colon. In reality the epithelium 

 is not a syncytium. 



In the more anterior part of the colon, the cross sections 

 of the Malpighian vessels and of the longitudinal muscles 

 are about equal in diameter. But as one traces the sections 

 caudad, the Malpighian vessels gradually increase in size while 

 the muscles decrease in size, and two thirds of the way to the 

 end of the colon this has become very marked, the Malpighian 

 vessels here being twice as large as the muscles. At this point 

 the longitudinal muscles begin to diminish in size rapidly 



