BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 311 



better arrangement for such an animal, which has only two muscular coats, to 

 have the longitudinal fibres internal, for, on the occurrence of rupture, they, as 

 well as the other tissues, are constricted by the circular ; whereas, in the supposed 

 arrangement of M. de Quatrefages, the longitudinal are beyond the reach of the 

 constricting belt. Other organs also in the same animal, such as the proboscidian 

 sheath and long posterior gland, have their circular fibres exterior to the longi- 

 tudinal. The actions of this muscular system are very varied, and include swim- 

 ming or floating on the surface of the water, an action performed, as in the Nudi- 

 branchiate mollusca, by aid of the mucous exudation, and not, as stated by M. de 

 Quatrefages, chiefly by the cilia. 



Anteriorly the body- wall terminates in a rounded snout — of the usual cuta- 

 neous textures, presenting in transverse section an areolar and granular appear- 

 ance, the soft contents of the areolae having for the most part escaped. The 

 aperture for the proboscis lies at the ventral border of such a section. Somewhat 

 behind this, but yet in front of the ganglia, a remarkable interlacement of fibres 

 (Plate IV. fig. 1), occupying almost the entire cephalic region, occurs. Powerful 

 bands of fibres (1) pass below both the buccal cavity and the tube for the pro- 

 boscis, meet, and cross each other in an oblique manner, forming afterwards, by 

 their divergence, extensive lateral connections ; indeed, it will be observed, that 

 towards the inner muscular layer the fibres just mentioned form a broad fan- 

 shaped arrangement. Some of the fibres (2) pass upwards by the side of the 

 central canal, and mingle with those descending from this region; while others 

 (3) curve downwards to the ventral wall. The fibres (4) that meet above the 

 central canal cross each other obliquely in the middle line, so as to form a firm 

 arch ; and, besides, there are some transverse fibres (5) that cross over the canal, 

 and spread out on each side. Other bands of fibres (6) slant downwards and in- 

 wards on each side of the cavity, and meet inferiorly. The arrangement of these 

 bands and fibres is so intricate, that each seems to blend with the other, and form 

 a continuous anastomosis of contractile meshes. In addition to these oblique and 

 radiating fibres, there is a powerful series of longitudinal fibres interwoven with 

 them in an intricate manner, besides the denser grouping (e) at the margin (which 

 indicates the inner muscular coat of the body), and the glandular masses in 

 the centre. It will be observed that the bands which pass beneath the central 

 canal are the most powerful, and afford a much greater resistance to the bulging 

 of the proboscis and its sheath than the superior fibres, so that in extrusion the 

 organ is mainly directed upwards. This will be understood by referring to Plate 

 IV. fig. 5, which represents a section of an animal which had protruded a small 

 portion of its proboscis after chloroforming and immersion in spirit. The inferior 

 commissure of the ganglion is thus somewhat protected by the arrangement of 

 the fibres in front of it. The blood-vessel (Plate IV. fig. 1, I) lies on each side in 

 a sheltered position, in an angle between two series of fibres ; and its calibre 



