12 bulletin: museum of comparative zoology. 



3Iusculati(re. 



In all nemerteans the body walls are provided with at least two 

 layers of powerful muscles. In the Paleonemertea and Hoplone- 

 mertea there are but two distinct muscular layers extending through- 

 out the whole length of the body, although one or more additional 

 layers may occur in some species of the Paleonemertea, in certain 

 portions of the body, as described below. The outer of these is 

 circular and the inner longitudinal. In the Heteroneniertea a third 

 distinct layer, consisting of longitudinal fibers, is placed outside the 

 other two layers. The musculature of Malacobdella is closely 

 similar to that of the Hoplonemertea. 



In Paranemertes californica the longitudinal musculature for 

 some distance behind the head consists of two distinct layers of 

 fibers (Text-tig. 6), separated by a thick layer of parenchyma. 

 They become united into a single musculature well back toward 

 the intestinal region. 



Diagonal muscles. — In numerous species belonging to each of 

 the principal orders, a double layer of diagonal muscles (PI. 12, fig. 

 79) is situated between the two principal muscular layers, where but 

 two are present, or between the circular and outer longitudinal lay- 

 ers in the ITeteronemertea where three layei's occur. The diagonal 

 muscles are nearly always very thin as compared with the thickness 

 of the principal muscular layers. 



In the cephalic region of the Heteronemertea and many of the 

 Paleonemertea, the arrangement of the muscular layers is broken 

 up to form a complex network of fibers running in all possible 

 directions, but of which the longitudinal fibers are the most nu- 

 merous. In other forms the two muscular layers extend forward 

 into the head. 



Inner circular muscles. — In many -species of Carinella, Mi- 

 CRURA, Zygeupolia, and especially in Carinomella and Caei- 

 NOMA,' in addition to the outer circular and the longitudinal 

 muscular layers, another layer — inner circular — is developed 

 internal to the longitudinal layer in certain portions of the body. 

 In a few species of Carinella this inner circular muscular layer 

 extends from the head to the posterior end of the body, but in other 

 species of the same genus, in Carinomella (PI. 7, figs. 55, 56), 

 and in Carinoma (PI. 13, fig. 82), this layer becomes enormously 



