COE: NEMERTEANS OF WEST AND NORTHWEST COASTS. 135 



The region of greatest thickening of these muscles is indicated in, 

 PI. 8. tig. 57, cm. 



For a short distance beliind the month the lateral blood vessels,, 

 with their sm-rounding parenchyma (PI. 6, fig. 54), lie inside the 

 inner circular muscles, but somewhat farther back in the esophageal 

 region they pass to the outside of these muscles, which now closely^ 

 invest the proboscis sheath and esophagus, as is the case also in 

 Carinella linearis. In Callinera, Bergendal (: 00) finds the inner 

 circular muscles to lie outside the lateral vessels both in the esopha- 

 geal and anterior intestinal regions, but this condition is very 

 unusual. 



In connection with the inner circular muscles, it should be noted 

 that in the esophageal region the nuclei of the muscle fibers are 

 usually nearly all congregated in the immediate vicinity of the lat- 

 ei'al blood spaces. Such a position is doubtless due to a more abun- 

 dant nutrition near the blood vessels than elsewhere. This circular 

 layer is also interrupted at intervals by outgrowths or diverticula of 

 the lateral vessels immediately between the latter and the rhyncho- 

 coel. These appendages to the blood vessels also pierce the wall of 

 the proboscis sheath t(? form the rhynchocoel vessels, as described 

 below. They are not very numerous, however, posterior to the region 

 where the lateral vessels pass outside the inner circular muscles. 



As stated above, the two circular layers often come closely in con^ 

 tact immediately above the proboscis sheath in the region of the 

 nephridiopores, and to these the circular muscles of the sheath itself 

 are closely approximated. For a ])rief space, therefore, no other 

 than circular muscles are encountered from the outside of the body 

 to the rhynchocoel in the mid-dorsal line (PI. 8, fig. 57). 



The very delicate outer circular muscular layer is bordered exter- 

 nally by a thin basement layer, except in the position of the lateral 

 and dorsal nerves which lie between the two layers throughout most 

 of the body. In the nephridial region, however, the lateral nerves 

 sink inward to the middle of the longitudinal nmscular layer (PI. 9, 

 fig. 60), apparently carrying with them a portion of the circular mus- 

 cular fibers. A few circular fibers, nevertheless, remain next the 

 basement layer, and are thus widely separated from those which 

 were carried inward by the lateral nerves. 



The outer circular muscles are interrupted in the position of the 

 lateral sense organs by a special set of muscles which originate in 



