Structure o/Magelona. 445 



sometimes of a symmetrical appearance, especially if one 

 fold of the proboscis is included within another. A base- 

 ment layer succeeds the foregoing, and then a coat of circular 

 fibres, -which attains its greatest development at the superior 

 or inner side (i.' e., next the snout in extrusion) and especially 

 near the base. There does not appear to be any special 

 longitudinal coat, other than the expansion of the long 

 retractor muscle on the walls near the extremity of the 

 extruded organ. At the latter region the thinner parts have 

 crossed fibres internally, an appearance probably due to the 

 commingling of the circular coat and the expanded fibres of 

 the long retractor. The latter forms a great muscular mass 

 which occupies the dorsal region of the organ in extrusion, 

 and therefore lies under the digestive canal on approaching 

 the body-cavity. It then is included between the anterior 

 fan-shaped retractors, wliich occupy the lateral region,^ and 

 thereafter proceeds along the ventral region of the body- 

 cavity to the termination of the anterior region. In trans- 

 verse section the muscle is divided into many large masses, 

 which are for the most part grouped in two divisions 

 superiorly, and a flattened band applied to the upper surface 

 of the transverse ventral muscle. The latter division — at 

 the point of differentiation of the transverse ventral into two 

 muscles — has become the most important, forming a compact 

 layer of elongated fasciculi (in section) ; while the other 

 divisions occupy the lateral regions in irregular masses and 

 in diminished bulk. Finally, behind the pharyngeal region 

 the lateral divisions disappear in the transverse sections — 

 probably by being attached to the superior raphe at each 

 side, — and only the ventral portion of the retractors remains 

 as a firm double muscle with symmetrically arranged fasci- 

 culi, situated in the central line over the transverse ventral. 

 As the muscle diminishes in bulk and approaches the central 

 line on each side, a firm granular mass is formed, first be- 

 neath it, then at each side from the vertical muscle inward, 

 and lastly above it — ^just before the fibres cease at the 

 ninth body-segment, and in front of the commencement of 

 the ventral blood-vessel. The importance of this muscle is 

 shown by its extensive connections, all of which could not 

 be readily destroyed at once, so as to interfere with the 

 boring. 



The foregoing structure enables the functions of the 

 organ in the economy of the animal to be understood. By 

 its agency the annelid can bore almost continuously forward 

 in the sand and fashion a tunnel in which the more delicate 

 posterior part reposes in safety. The contraction of the 



