UROGENITAL SYSTEM OF MYXINOIDS 115 



or near the middle of the somite. Those of a, side are not sepa- 

 rated from each other by equal intervals, but, as a rule, are 

 arranged in pairs, a greater distance occurring between two pairs 

 than between the two bodies of a pair (figs. 55 and 56). This 

 paired condition is in some way connected with the arterial 

 supply, as is shown by figure 56. Each of the Malpighian bodies 

 represented in this figure is in a separate somite. The two 

 arteries which supply the two Malpighian bodies of one of these 

 pairs are usually branches from, a somatic artery, as shown by 

 figure 56, but sometimes one of the arteries arises directly from 

 the dorsal aorta, close beside the somatic artery. When a 

 Malpighian body is not one of a pair, its artery comes directly 

 from the dorsal aorta. Seldom are the two Malpighian bodies 

 of corresponding somites on each side of the body opposite each 

 other, as is shown by figure 55. 



The Malpighian bodies and their tubules usually extend ante- 

 riorly to the point where the tubule empties into the mesonephric 

 duct. When, however, each Malpighian body in two adjacent 

 somites is supplied by a branch from the same somatic artery, 

 the anterior one is doubled back upon its tubule so that the 

 distal end points posteriad toward the origin of the artery 

 which supphes it. 



Each Malpighian body has a tubule which enters the meso- 

 nephiic duct. There are two types of these tubules which differ 

 distinctly and which will be designated as 'plain' and 'compound' 

 tubules. All the tubules of a given animal are of one type 

 only. 



The plain tubules are composed of very short, columnar cells 

 which have very little cytoplasm, as shown in figure 57. These 

 cells are all approximately the same size. Their nuclei stain 

 much more deeply than either the nuclei of the connective tissue 

 or those of the mesonephric duct, hence the tubules are very 

 distinctly differentiated from surrounding tissues. At the 

 proximal end the cells of the tubule are directly continuous with 

 those of the mesonephric duct, and at the distal end with the 

 cells of the single layer of squamous epithelium which lines 

 BowTiian's capsule. The basal membrane of the mesonephric 



