coe: nemerteans of west and northwest coasts. 13 



thickened in the nephridial region, or immediately anterior to the 

 intestinal region, and then ends abruptly at the beginning of the 

 intestinal region, or continues backward merely as a few scattered 

 libers. 



Where the inner circular muscle is developed, crosses between it 

 and the outer circular layer are often formed on the dorsal side of 

 the body, and sometimes also on the ventral side (PL 13, fig. 81). 

 These muscular crossings are especially well developed in Cari- 



NELLA. 



In Atnjyhiporus punctatuhis and to a less extent in Taeniosotna 

 punnetti a conspicuous layer of inner circitlar muscles closely invests 

 the posterior portion of the rectum (PI. 21, fig. 134) and forms a 

 well-marked anal spliincter. 



In Zygeupolia and in certain species of Micrura this inner cir- 

 cular muscle is found only for a short distance immediately anterior 

 to the intestinal region, where it reaches a considerable thickness, 

 and then suddenly ceases, exactly as in Carinoma. 



Dor so-ventral muscles. — In all nemerteans a few radial and 

 dorso- ventral muscular fibers are to be found in almost all parts of 

 the body. The dorso-ventral muscles are especially well developed 

 in the intestinal region of Cerebratulus and other forms which 

 are very much flattened posteriorly. In such species they form dis- 

 tinct, flattened bands, placed perpendicularly to the axis of the body, 

 and extending between the ventral and the dorsal portions of the 

 body musculature. These bands alternate regularly with the intes- 

 tinal lobes and extend fi-om the periphery of the lobe to the axial 

 tube of the intestine, thus corresponding in width to the depth of 

 the intestinal lobe. With the develoi)ment of the sexual glands, the 

 dorso-ventral nmscles are split vertically, the sexual glands develop- 

 ing between the two portions, showing that each dorso-ventral band 

 is in reality composed of two distinct bands of fibers. 



In Amphiporus 2)i^)ict(:ituhiSj.whic\i is much flattened and adapted 

 for swimming, strong bands of such mxTscles occur throughout the pos- 

 terior half of the body, and are particularly well developed in the 

 region of the rectum (PI. 21, fig. 13'2). 



In all forms, moreovei", in which intestinal lobes are developed, 

 there are more or less distinct bundles of these dorso-ventral muscu- 

 lar fillers alternating with the intestinal lobes. This dorso-ventral 

 musculature is probably homologous with the inner circular muscle 

 of the Paleonemertea. 



