136 bulletin: museum of comparative zoology. 



the longitudinal layer, and which pass through the circular muscles 

 to furnish special movement and contraction of the sense organ. 



Delicate muscular crosses between the two circular muscular 

 layers occur on both the dorsal and ventral sides of the body. The 

 dorsal crossing is much the better developed and increases in 

 strength from the mouth to the nephridial region. In the region 

 of the nephridiopores this crossing becomes sufficiently strong to 

 bring the two layers of circular muscles into contact, and thus cause 

 a marked infolding of the dorsal surface of the body in the median 

 line in this region (PI. 8, fig. 57). This infolding can often be 

 detected by means of a hand lens on the body of the worm. When 

 the fibers of the crossings are less strongly contracted the infolding 

 is naturally less than that shown in the drawing (Fig. 57). The 

 dorsal crossing is very delicate in the intestinal region, although it 

 can be traced throughout the whole extent of the body. The ven- 

 tral crossing is much less marked, but it is easily recognized as far 

 back as the nephridiopores. Posterior to this region it is only 

 faintly indicated, and cannot be detected beyond the anterior por- 

 tion of the intestinal region. 



The very powerful band of longitudinal muscles which is so highly 

 developed between the proboscis sheath and the alimentary canal 

 posterior to the thickening of the internal circular muscles is very 

 characteristic. It is so intimately connected with the proboscis 

 sheath more anteriorly that its description will be found in connec- 

 tion with that of the proboscis sheath (p. 129). It is separated from 

 the other longitudinal muscles of the body by the delicate internal 

 circular muscles only (Pi. 8, fig. 58; PL 9, fig. 59). A somewhat 

 similar muscle occurs in several species of Carinella, and is 

 described by Bergendal (: 00, p. 110) for Callinera, and (: 04, 

 p. 53) for Carinoma, where it is comparatively little developed, 

 while in Carinella linearis the conditions are much as in the present 

 species. 



The basement layer is everywhere very thin, and in most instances 

 is hardly to be distinguished from the outer circular muscles, except 

 in the head and esophageal regions. Farther back the two layers 

 together appear only as a single thin layer, except under the higher 

 powers of the microscope. It is best developed in the brain region 

 and becomes gradually thinner toward the posterior end of the 

 body. It is also well developed in the vicinity of the lateral sense 



