218 MAGELONA. 



closely arranged papillae ; proboscis extrusible ; complex vascular system with pink 

 corpuscles. Body of three divergent regions, the middle of only a single segment. No 

 branchiae. Two anal cirri. 



In the structure of the snout Magelona offers features sni generis, but, as they have 

 been already minutely described, 1 only a brief epitome is necessary here. The snout is 

 remarkably flattened, eyeless, translucent, and thinned at the margins like a sharp -edged 

 spatula. It is covered with cuticle and hypoderm, and beneath has a median and a 

 lateral pair of longitudinal muscles, which are supported by a chitinoid basement-tissue 

 in the form of a crown, the whole giving firmness to the snout in its ceaseless thrusts 

 into the sand, aiding its muscles, and confining their actions to the most favourable 

 lines. Moreover the flexible chitinoid plates — even more than the beams of a ship — lend 

 the necessary power of resistance to the yielding tissues of the snout and protect the 

 blood-vessels. The median muscles cease when they reach the line of the tentacles. 

 The lateral muscles take origin a little behind the median, form ribbon-shaped bands, 

 become narrower toward the mouth, and are continued backward to the tail as the 

 ventral longitudinal muscles. A powerful transverse muscle in front of the mouth 

 connects the inferior ends of the external chitinoid septa, and the space is further 

 subdivided by vertical fibres. The powerful band is the chief agent in narrowing the 

 snout. 



The dorsal longitudinal muscles take origin between the forks of the chitinoid process 

 over the mouth as narrow bands, wider as far as the sixth setigerous process, then 

 diminish to the ninth, and again spread out thereafter, being at first separated by an 

 intermediate hypodermic process which passes downward to the apex of the oblique 

 muscle and the alimentary and vascular systems beneath. Then a simple sulcus is left, 

 from which the fasciculi in transverse section pass off in a pennate manner, and this 

 continues posteriorly, only disappearing before the termination of the muscle at the tip 

 of the tail. Externally a thin layer of circular fibres envelops the body-wall beneath the 

 hypodermic basement-tissue, and is continuous to the posterior end of the worm. 



In the oral region the muscular system is complex, and amongst others the transverse 

 dorsal muscle extends from the anterior border of the mouth to a little behind the 

 tentacular bases. Some of the vertical fibres are attached to the proboscis, others to 

 the chitinoid plates in company with the large transverse ventral muscle. The whole 

 body-cavity, by the aid of these muscles, can be firmly compressed, and the proboscis 

 expelled. Both vertical and oblique muscles attain great development throughout the 

 entire anterior region, so as to act as powerful compressors, whilst their elongation and 

 contraction directly affect the blood-channels. Moreover, immediately in front of the ninth 

 segment the relations of the muscles are unchanged ; but it is now observed that a series 

 of muscular fibres arise from the superior insertion of the vertical, outward along the 

 whole upper arch of the body, and indeed to a point considerably below the upper fibres 

 of the oblique. They by-and-by form a powerful fan-shaped mass extending from the 

 raphe of the dorsal longitudinal muscles, and even over them, to the edge of the 

 ventral longitudinal muscles inferiorly, and converging to the raphe at their inner border. 

 Some fibres also pass into the transverse ventral muscle so as to form a continuous 

 1 'Zeitschr. f. wiss. Zool./ Bd. xxxi, p. 401, pi. xxix — xxxviii, 1877. 



