BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 313 



animal, sometimes almost clasping the elongated proboscis, at others being 

 attenuated over the doubled organ. 



The various transverse sections of the worms also render the relations of the 

 cavity more apparent. Like the proboscis, its anterior end passes through the 

 ring formed by the arching of the superior commissure, the inferior commissure, 

 and the sides of the ganglia. The nervous matter must thus occasionally undergo 

 very great stretching, or else the proboscis is rarely launched out. This will be 

 more particularly noticed in the description of the ganglia, and a reference to 

 Plate IV. fig. 5, will suffice in the present instance. The inferior commissure 

 separates it entirely from the chamber of the great ciliated oesophagus. The rela- 

 tion of the parts in the ganglionic region is represented in Plate V. fig. 1, o being 

 the wall of the proboscidian sheath somewhat compressed, so as to show both 

 longitudinal and circular fibres ; for it may be mentioned, that the structure of 

 the chamber wall is powerfully muscular, as evinced by its ever- varying condition. 

 At this point, however, the fibres have not attained a great degree of develop- 

 ment. In a section made further back (as in Plate V. fig. 2, o), and in the other 

 transverse sections, this muscularity is more distinctly exhibited, though, of 

 course, the spirit has shrivelled all the parts, especially the muscular. Exter- 

 nally the wall of the chamber is furnished with a layer of circular, and inter- 

 nally with a series of longitudinal fibres, both becoming thinner posteriorly. 

 The comparatively large size of the cavity during life has doubtless caused several 

 observers to err, by confounding it with the supposed general cavity of the body. 

 The presence of ova or sperm-sacs has a considerable influence in modifying the 

 size of the chamber, which in the ripe animal is pressed upwards and towards the 

 median line, while in the spawned worm it expands freely in all directions. It is 

 a mistake, however, to suppose, with M. de Quatrefages, that no cavity exists pos- 

 teriorly in the ripe animal, for this chamber holds the same anatomical relations 

 from the ganglia to the tail as at other seasons, only its calibre is encroached on 

 posteriorly, and the consequent distention by the proboscis and fluid makes it 

 more conspicuous in front. The chamber is absent in the aberrant form Polia 

 involute/,, Van Beneden. 



In the foregoing cavity the proboscis floats in a clear fluid, rich in large 

 flattened discs, which have a minutely granular appearance. In the living 

 animal, these generally have a fusiform outline, from a slight thickening in the 

 middle (Plate IV. fig. 9, b). They are accompanied by certain granules and globules, 

 which are also represented in this figure. The discs vary in size, and adhere to- 

 gether in a dying animal very easily, from the highly coagulable nature of the 

 transparent fluid in which they float ; and occasionally fibrinous shreds may be 

 observed attached to them under the same circumstances. The fluid, indeed, is 

 highly organised, and very different from sea- water, to which Dr T. Williams 

 compares it. When the proboscis has been gently protruded under chloroform, 

 vol. xxv. part ii. 4 L 



