Structure of Magelona. 4;U 



at first appears iu transverse section as a small band, lying 

 chiefly below the median muscle, and having the vascular 

 area superiorly and externally. When the crown-shaped 

 arrangement of the central muscles occurs, the external (or 

 lateral) have attained a somewhat larger size, and their fine 

 parallel fasciculi in section are placed in a concentiic 

 manner round tlio chitinous loop. Each muscle therefore 

 forms a hollow process in the snout, and in the I'resh 

 specimen, under compression, granules and cells rush to and 

 fro in the median space. 'J be latter, in the preparations, 

 hohls lax ceHulo-granular tissue — also somewhat concentri- 

 cally arranged. The septum between it and the median 

 muscle is, for the most part, occupied by the vascular area 

 and presents no fasciculi, so that the muscle thus has the 

 form of the letter U, the tliickcst mass occurring superiorly, 

 and the thinnest at the curve of the U externally. At the 

 tip of the U superiorly the fasciculi are directed downward 

 and outward, and inferiorly upward and more obliquely 

 outward. Proceeding backward the area of the muscle 

 largely increases, chiefly toward the ventral surface, but the 

 general direction of its fasciculi is the same. Before the 

 separation of the muscles in front of the mouth the area, 

 again, considerably diminishes, especially in regard to the 

 lax central tissue. Immediately after the separation, the 

 gap caused by the passage of the vessels under the central 

 area is filled by the latter tissue, and the limbs of the U 

 approach each other more closely at the tip, but they do not 

 unite. In a line with the base of the tentacles the small 

 rounded area of each lateral muscle in section is almost 

 entire ly occupied by the fasciculi, which are nearly horizontal 

 in direction, the npper, however, inclining downward and 

 the inferior upward. There is now a tendency toward a 

 ventral position, the superior curve of the muscle disappearing 

 and the inner ventral region increasing in size. Behind the 

 tentacles it is almost vertical, with the fasciculi for the most 

 part directed transversely, the convex chitinous support 

 being external — its superior edge running into the base- 

 ment-tissue under the dorsal hypoderra ; while the inferior 

 border terminates at the insertion of the oblique muscle (in 

 this region vertical) which forms its inner border and shuts 

 it iu a special ctjm|)artment. In the concavity of the 

 muscle is still found a little of the lax cellulo-granular tissue. 

 The muscle gradually enlarges and moves dosvnward so as to 

 get beneath the bristles and toward the infero-lateral regiot!. 

 About the ninth body-segment it becomes chiefly ventral 

 in position, then rapidly diminishes in size at this segment, 



