S(ructu7'eof^la.^ii\ona.. 447 



externally ; while certain longitudinal fibres whicli appear 

 in the latter become by-and-by incorporated with its wall, 

 which externally sliows traces of longitudinal fibres, outside 

 the circular, though they are difficult to discern. Tlie 

 relations of the organ with the vascular system at this part 

 are, indeed, complex. The form of the alimentary canal in 

 section near the ninth body-segment is now rounded or oval, 

 with a smooth outline, and instead of the few bold folds of 

 glandular tissue covered by the chitinous layer internally, 

 the glandular substance is granular and projects in conical 

 elevations or papillai having a dendritic or radiated appear- 

 ance. As the canal diminishes the latter become more 

 pointed and translucent, the basement-tissue more pro- 

 minent, and within the circular coat are distinct traces of 

 a layer of longitudinal muscular fibres. At the ninth body- 

 segment the circular coat is much thickened, the basement- 

 tissue increased, and the glandular layer more consistent, so 

 that it projects inward very distinctly. The canal reaches its 

 minimum diameter about the termination of the transverse 

 ventral muscles, nea* the commencement of the ventral 

 blood-vessel. Just at the point where the dorsal longitudinal 

 muscles begin to expand, the tube is surrounded by a 

 powerful ring of muscular fibres, which in the mid-ventral 

 line join a transverse chitinous raphe, from the under 

 surface of whicli spring the strong muscular walls of the 

 ventral vessel of the region. The canal has now increased 

 a little in size, but is often squeezed (in the preparations) by 

 the pressure of the superincumbent blood-vessels, so that it 

 forms a transverse slit. It then passes the boundary-line 

 into the succeeding region, and will fall under notice 

 shortly. 



The anterior region of the alimentary canal is distinguished 

 as a whole by its greater muscularity and firmness, by the 

 massive nature and chitinous covering of its glandular coat, 

 and by the absence of evident capillaries from its walls. It 

 presents a close homology with the Nemertean oesophageal 

 region, especially in its relation to the circulatory fluid, for 

 in the latter a 7-ete mirabile likewise occurs. The minute 

 structure of the wall is similar, but, as on the skin, the 

 chitinous tunic internally in Magelona takes the place of the 

 cilia in the Nemertean. Further, like the Nemertean organ, 

 it retains irritability long after the death of the animal, and 

 in dissection is often torn from the more fragile tissues of 

 the anterior region as a tough thread. The functions of the 

 parts, in regard to digestion, in the respective groups are 

 probably similar, and may represent all the complex divisions 



