256 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



proboscis sheath and beyond the blood vessels. As shown in PI. 

 21, fig. 132, they here lie in the midst of the body parenchyma, 

 about half way between the rectum and the circular muscles. Their 

 union above the rectum is very conspicuous and lies some little 

 distance posterior to the anastomosis of the blood vessels. 



At the posterior end of the intestiiie proper (or mid-gut) its lateral 

 lobes quite disappeai", and the short rectum following has a very 

 small lumen (PI. 21, figs. 182, 134). In this region a very consid- 

 erable number of circular muscular fibers appear, surrounding the 

 rectum (PI. 21, Text-fig. 132), and these increase so greatly in num- 

 ber farther back as to form a marked inner cu-cular layer (PI. 21, 

 figs. 133, 134). The extent of these muscles is short, for they first 

 appear as a distinct layer near the posterior end of the rectum. 

 They are, however, of considerable interest, for they seem to be 

 homologous with the inner circular layer which is developed about 

 restricted portions of the alimentary canal in some other nemerteans 

 of widely different orders. In Micrura, for instance, and especially 

 in Zygeupolia (Thompson, :02) a distinct layer of circular muscles 

 is found closely fitting about the posterior end of the esophagus ; in 

 Carinoma a similar layer becomes massively developed in a corre- 

 sponding region. 



As has been stated for Micrura alaskensis (Proc. Wash. Acad., 3, 

 p. 73, 1901), I am disposed to consider all these muscular thicken- 

 ings as merely excessive development in localized areas of the 

 extremely thin and delicate muscular tunic (in many instances diffi- 

 cult to demonstrate) which extends throughout the length of the 

 alimentary canal in many, if not all, species of nemerteans. In the 

 present species the thickening of these muscles occurs at the pos- 

 terior end of the rectum, instead of at the posterior end of the 

 esophagus as in the other forms mentioned. 



Just at the anastomosis of the lateral nerves this inner circular 

 (or intestinal) muscle becomes several times as thick as the main 

 circular layer of the body walls. Behind the nerve anastomosis all 

 the muscular layers become interrupted and intermixed with large 

 numbers of oblique fibers. But still the inner circular muscle is 

 clearly distinguishable from the main circular la\'er (PI. 21, figs. 133, 

 134) and is several times as thick. At the nerve anastomosis the 

 parenchyma of the body cavity is discontinued, and behind this 

 point the muscles fill all the space between the integument and the 



