Structure of Magelona. 439 



shortly fusiform or almost baccate ia contraction, especially 

 toward the termination of the region, the median ventral 

 sulcus being very deep. At the ninth body-segment many of 

 the fibres, as formerly noticed in connection with the great 

 fan-shaped muscle of the part, seem to pass right across the 

 median septum, from one muscle to- the other. They 

 diminish much immediately thereafter, so that at the com- 

 mencement of the greenish intestine in the next segment 

 only their form is indicated by two small swellings on the 

 band between the longitudinal ventral muscles. Finally, 

 the chititious band into which they degenerate separates the 

 ventral blood-vessel from the nerve-cords, and externally 

 gives attachment to the vertical and oblique muscles. This 

 donble-bellied condition of the atrophied remnant of the 

 transverse muscle is continued to the posterior end of the 

 worm — little more than the chitinous tissue (which in front 

 supported the muscle) being left. 



The muscular arrangements of the anterior region of the 

 body render the presence of dissepiments unnecessary, so 

 that the first occurs at the commencement of the posterior 

 division, and they are continued in each segment to the tail. 

 Each forms a muscular screen situated a short distance 

 behind the lamellae and hooks, and, while permitting the 

 passage of the perivisceral fluid, serves to support the 

 intestine and the blood-vessels. 



The bristle-muscles are on the whole feebly developed and 

 somewhat indistinct, but they seem to agree with those in 

 allied forms. The roots of the superior bristles of the 

 anterior region pass obliquely inward to the upper part of 

 the vascular space between the vertical and oblique muscles, 

 and the special slips radiate outward to the hypoderm. The 

 roots of the inferior, again, abut on the upper part of the 

 hollow of the ventral longitudinal muscle. Their special 

 slips follow a similar arrangement. Those of the ninth body- 

 segment have homologous relations with surrounding parts, 

 but they form a much larger fan-shaped transverse series, 

 the diminution of the dorsal and ventral longitudinal muscles 

 affording ample space for their expansion. In the posterior 

 region of the body the great increase of the longitudinal 

 muscles just mentioned causes the hooks to assume a more 

 or less vertical position. Their muscles are similar to those 

 moving the anterior bristles ; and as they are on elevated 

 hypodermic processes continuous with the lamellee, both are 

 probably acted on thereby. 



The foregoing muscular system enables the animal to 

 perforate the moist sand with great rapidity, as well as to 



