Structure o/'Magelona. 433 



lateral (external) muscle ; and it is clear that this powerful 

 band is the chief agent in narrowing the snout from side to 

 side. Proceeding backward it is found to increase in 

 strength, while the vertical fibres in the inferior space 

 diminish (a few occurring laterally). Then a stronger 

 vertical series, springing from the middle of the chitinous 

 arch superiorly — between two lateral channels — and spreading 

 out in a fan-like manner through the powerful transverse 

 plate above mentioned, reaches the ventral hypoderm with 

 its base and obliterates the inferior space. The two muscles 

 jire evidently antagonistic in action, and their important 

 functions in connection with the vascular system may be 

 further exemplified by contrasting specimens in which the 

 region is contracted with those in wdiich it is largely dilated. 

 'J'he inner wall of the chamber being composed of elastic 

 chitinous tissue, very great expansion is permitted. In 

 longitudinal sections the transverse muscle is well seen in 

 its space just in front of the mouth, with the vessels at its 

 upper border. The space or cavity is comparatively short 

 (antero-posteriorly) ; indeed, it is confined to the pre-oral 

 region. Further, the vertical fibres from the chitinous 

 superior arch seem to meet over the transverse muscle and 

 send bundles in front of and behind it — the latter being the 

 stronger. The fibres of the vertical muscle are attached to 

 a chitinous plate, which springs from the anterior border of 

 the superior transverse muscle, beneath which the dorsal 

 blood-vessels pass. The latter muscle in some preparations 

 is narrow and deep in the middle^ and spreads out at each 

 side. 



The next muscles that come under notice are the longi- 

 tudinal dorsal, which, when viewed from above, take origin 

 between the forks of the chitinous process over the mouth as 

 narrow ribbons, widen till about the sixth set of bristles, 

 then diminish to the ninth, and again spread out thereafter. 

 In section they are found to commence in front of the 

 vascular space as two small slips surrounded by the usual 

 chitinous basement-tissue situated over the transverse 

 muscle. They then pass below the latter, increase in size, 

 and form the superior arch of the chamber for the dorsal 

 blood-vessel. Proceeding backward they gradually extend 

 outward and enlarge — tlie deepest part of each being toward 

 the middle of the muscle, and the thinnest near the mesial 

 line, "where a raphe occurs. Externally is the hypodermic 

 basement-tissue, internally (in the median line) the dorsal 

 "blood-vessel and the esophagus at the sides ; laterally each 

 abuts on the origin of the external lateral muscle. Toward 

 Ann. & Mag. N. Hist. Ser. 8. Vol vii. 29 



