BRITISH NEMERTEANS. AND SOME NEW BRITISH ANNELIDS. 319 



The anterior fibres of the proboscis, as further shown in the various transverse 

 sections, and in the ruptured organ when extruded, are chiefly longitudinal, and 

 while the thinness of the coats renders the exact structure of this region in trans- 

 verse section less distinct, a very definite arrangement is observable as soon as 

 the tube has attained larger proportions. Dr Johnston, indeed, considered the 

 organ to be homogeneous ;* and M. de Quatrefages describes its commencement 

 in Polia mutabilis as consisting of two longitudinal muscular coats, separated from 

 each other by a cellular layer, which, he explains, is a provision for enabling these 

 muscular coats to act independently. He also observes, that no circular fibres 

 were seen in this species, in P.Jllum, and some others. In very small specimens 

 of the British examples the transparency of the tissues renders definition of the 

 coats somewhat obscure, especially after mounting in chloride of calcium, but, so 

 far as I have observed, the structure is as follows : — Externally, there is a layer 

 of what appears to be elastic tissue (Plate IV. fig. 4, g, Plate V. fig. 4, g, &c). 

 It is more distinctly striated in transverse than in longitudinal sections of the 

 organ, hence it may be inferred that its fibres are chiefly circular in direction, 

 as seen on comparing the last-mentioned figures. Towards its free border, also, 

 certain obscure granular markings observed in the longitudinal section (Plate IV. 

 fig. 4), show that the direction of the external fibres is different from the others ; 

 indeed, in some views, the appearance is such as to raise a suspicion of the 

 presence of the cut ends of a few fine circular muscular fibres, the rest being 

 nearly homogeneous. Within this is a somewhat narrow belt of longitudinal 

 muscular fibres (/, same figures), which may be termed the external longitudinal 

 muscular coat. It consists of pale, unstriped muscular fibres, whose cut ends are 

 seen in Plate V. fig. 4. Intervening between this coat and the other longitudinal 

 layer is a remarkable stratum, the reticulated or beaded layer (0), in the same 

 figures, which in transverse sections (Plate V. fig. 4) assumes a regularly monili- 

 form appearance, from an increase of its constituent substance at certain points. 

 In longitudinal sections, I was for a time puzzled by the appearance of the cut 

 ends of fibres in this layer, as if it had been composed of circular fibres ; and a 

 more minute examination showed that such was due to certain intermediate bands 

 which passed between the thicker or beaded portions. If a thin longitudinal slice 

 from the organ in 0. pulchra is hardened and mounted in chloride of calcium, 

 numerous well-marked homogeneous longitudinal belts are seen at regular 

 intervals, from one end of the anterior region of the proboscis to the other, and 

 between them are many connecting transverse fibres, which pass from each edge 

 of the belt. The cut ends of the fibres in the longitudinal sections have therefore 

 been caused by the knife severing the transverse meshes between two longitu- 

 dinal belts. Thus the tube is surrounded by a complete investment of this 



* Catalogue Brit. Museum, p. 285. 



