COE: NEMERTEANS OF WEST AND NORTHWEST COASTS. 255 



Body vmlls. — The posterior portions of body are very much 

 flattened dorso-ventrally (PL 21, tigs. 132, 138), thereby enabUng 

 the worms to swim readily. Tliis flattening is brought about by a 

 great development of dorso- ventral muscular flbers passing from the 

 dorsal to the ventral portions of the body walls between the intes- 

 tinal lobes, as in Cerebratulus. These muscles are found to con- 

 tinue to the very extremity of the body ; posteriorly to the intestine 

 proper they lie between the rectum and the lateral nerves (PI. 21, 

 flg.132). 



A distinct set of diagonal muscular flbers lies just inside the circu- 

 lar muscular layer of the body walls, as in some other species of the 

 genus. They run obliquely to the other muscles of the body, and 

 in reality are disj^osed in two separate layers of fibers which are 

 directed at right angles to each other, as are the diagonal muscles in 

 Cerebratulus, Carixoma, and other forms. 



These diagonal muscles are most conspicuous in sagittal sections 

 of the body, especially in the esophageal region, and the fact that 

 the two sets run at right angles is here very evident. In cross sec- 

 tions, however, they are by no means diflicult to observe, for they 

 are separated from the circular muscles by a thin sheet of connective 

 tissue, somewhat as in Carinoma mutabilis (PI. 12, fig. 79). 



The individual muscle bundles of the longitudinal muscular layer 

 are well separated in the esophageal region, and especially above 

 and below the lateral nerves, by bi-oad outgrowths of the connective 

 tissue of the body cavity which reach outward quite to the circular 

 muscular layer. 



Posterior end of body. — The greatly flattened posterior extremity 

 shows some interesting peculiarities, which should be described in 

 detail. The proboscis sheath retains its tliick walls until near the 

 end of the body. It then becomes very small, its lumen gradually 

 disappears, but its musculature is prolonged some little distance far- 

 ther back, and is eventually lost in the longitudinal muscles of the 

 body walls. 



The dorsal blood vessel remains beneath the proboscis sheath 

 throughout the length of the latter, l)ut a little farther back enters 

 into a broad and conspicuous anastomosis with the pair of lateral 

 vessels at then* extremities. This anastomosis lies above the intes- 

 tine, as figured for A. angidatus (Coe, : 01, PI. 13, fig. 3). 



The lateral nerves remain of large size beyond the end of the 



