Structure of Magelona. 437 



longitudinal dorsal and the region external to it, and pass 

 obliquely downward to be inserted at the inner border of the 

 ventral. The oblique arise from the wide region between 

 the upper (external) border of the longitudinal ventral 

 upward to the superior lateral region, and the fibres extend 

 downward to the outer part of the common insertion — close 

 to the inner border of the longitudinal muscle. Shortly 

 after their attachment, as described, to the basement-tissue, 

 the arrangement of both is somewhat interfered with by the 

 very large fasciculi which pass transversely outward from 

 the sides of the alimentary canal to the body-wall (in the 

 extruded state of the proboscis). The fibres of the oblique, 

 again, at this part seem to run superiorly into the largely 

 increased lateral vertical. Immediately behind, the vertical 

 appears as a nearly cylindrical muscle (in transverse 

 vertical section), and the vascular channel lies between it 

 and the oblique. Masses of the great fasciculi (from the 

 alimentary canal) still occur at the superior and inferior 

 attachments of the vertical muscles, and probably represent 

 the retractor muscles of that part of the pharyngeal region 

 which is thrust into the proboscis. By the gradual passage 

 of the longitudinal ventral muscle downward, the inferior 

 fibres of the oblique become more horizontal, and both 

 A'ertical and oblique muscles attain great development 

 throughout the entire anterior region of the body, so 

 as to act as powerful compressors, while their elongation 

 and contraction directly affect the blood-channels between 

 them. On approaching the ninth body-segment the lower 

 fibres of the oblique are nearly horizontal, and both this 

 muscle and the vertical are large ; indeed, as the dorsal and 

 ventral longitudinal muscles diminish in bulk, the former 

 increase. Immediately in front of the last-mentioned seg- 

 ment the relations of the muscles are unchanged. It is then 

 noticed that a series of muscular fibres arise from the 

 superior insertion of the vertical, outward along the whole 

 upper arch of the body, and, indeed, to a point considerably 

 below the upper fibres of the oblique. These by-and-by 

 form a powerful fan-shaped mass sweeping continuously 

 from the raphe at the dorsal longitudinal muscle, and at 

 first even extending OA^er the latter, to the edge of the 

 longitudinal ventral inferiorly, and converging to the raphe 

 at its inner border. In some preparations fibres are observed 

 to pass into the transverse ventral muscle so as to form a 

 continuous band. The direction of the fibres of this great 

 muscular expansion is not straight or vertical, but they 

 curve from before backward,, as well as from above down- 



