Structu7-e of Magelona. 443 



special muscular arrangements well adapt it for such a 

 function. The exsertile portion, forming a distinct fold, 

 occupies the dorsal region, and is characterized, in addition 

 to its muscular development, by having the inner surface of 

 its floor thrown into very prominent transverse rugse, which 

 in the uninjured animal appear externally as well-marked 

 l;ars. In extrusion a great curve takes place behind the 

 buccal region, downward and forward into the base of the 

 exserted proboscis ; yet, though the folds are boldly defined, 

 it is difficult to follow their windings, or to estimate their 

 exact relations with the parts in the retracted condition. 

 Further, it occasionally happens that the great muscular 

 plexus of the protrusible part is inverted, i. e., is situated 

 dorsally instead of veutraliy. The region supplied with the 

 well-marked muscular coat is much dilated in the interior of 

 the proboscis, and this special disposition will materially aid 

 in the withsh-awal of the organ to its position in the body. 

 In longitudinal section the circular muscles of the pharyngeal 

 coat are observed to be clasped between the radiating series 

 in linear spaces, while the diverging form a very regularly 

 looped system of crossed fibres, which probably have a some- 

 what spiral arrangement (and the indistinctness of the 

 longitudinal fibres in transverse section woiild thus be 

 accounted for). The chief muscuhir retractor, attached 

 posteriorly to the superior raphe on each side, is well shown, 

 as is also a band of fibres from the latter raphe, which passes 

 backward for insertion at the anterior part of the extruded 

 organ, and which would pull it inward and forward in 

 retraction. Anteriorly, again, fibres proceed from the 

 anterior fold of the organ (ventrally) forward to the fold of 

 the buccal glandular tunic, where it joins the proboscis ; and 

 in the extrusion of that tissue the anterior region of the 

 pharynx would be pulled on, thus explaining, in all probability, 

 the inversion of the latter, which sometimes, as above 

 mentioned, shows its muscular coat superior in the extruded 

 region. 



The expulsion of the proboscis is doubtless the primary 

 agent in the movement of this pharyngeal region, then 

 follow the dragging on the anterior part by the ventral 

 fibres and the action of the body-walls of the animal. In 

 withdrawal, again, its great retractors, and the contraction 

 of the expanded special coat, with its longitudinal, looped^ 

 and other fibres, would suffice to bring it into position. 



The Proboscis. 

 This organ in life is thrust out at somewhat regular 



