BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 375 



stratum (3, 3), which in transverse section (fig. 6) has a transversely barred 

 arrangement with linear interruptions, which divide it into numerous and some- 

 what regular elongated spaces. In longitudinal section, again (fig. 7), this 

 stratum has a wavy aspect, or, if much contracted, presents a series of moniliform 

 streaks. That this layer, however elastic, is not muscular, a glance at the 

 position of the parts in fig. 7 at once demonstrates. It belongs entirely to the 

 cuticular elements, and with the interior pigmentary layer corresponds to the 

 region d" in B. olwacea, which, in the larger species, attains much greater per- 

 fection, and becomes distinctly separated from the longitudinal muscular fibres. 

 The only peculiarities in the muscular coats consist in the very evident transverse 

 streaking of the external longitudinal layer (fig. 7, e), and in the presence of 

 certain parasitic (?) cellular masses in it and the next outer layer. These masses 

 lie in definite spaces, and consist of groups of rounded cells filled with granules. 

 In the contracted state of the animal, as after preservation in spirit, the fibres of 

 the circular coat in longitudinal sections are grouped in a wavy manner (e\ fig. 7), 

 apparently from the extreme shortening of the parts. 



In the arrangement of the muscular system of the body-wall the curious 

 specimen from Balta is distinguished from all other British forms yet encoun- 

 tered. Externally (Plate X. fig. 2, d'), beneath the basement-layer of the cutis 

 (which in the fragmentary specimen was almost absent), there is a layer of cir- 

 cular fibres (ef). Within the latter is a very powerful layer of longitudinal fibres 

 (<?), which (layer), however, is not continuous, as in Ommatoplea and Borlasia, 

 but has at least one very distinct point of separation. Upon approaching the 

 middle line of the dorsum in transverse section, this longitudinal coat becomes 

 thinned off, so as to end on each side of the centre in a blunt point. In addition, 

 there is a somewhat triangular portion (ea) cut off by interfascicular substance 

 and fibres. The dorsal curve of the proboscidian sheath is closely applied to this 

 central point of separation, apparently receiving therefrom a few fibres, which 

 retain it in position, while other fibres pass downwards to join the circular layer 

 (Ja), which here encloses the space for the digestive tract. The separation of the 

 great longitudinal layer of the body-wall is marked externally by a distinct 

 median line, which is rendered more conspicuous by the occurrence of the trans- 

 verse strise of the dorsum on each side of it. There is also a slightly marked 

 fissure of this muscular coat inferiorly. This arrangement therefore conforms to 

 the Meckelian type, as seen in M. annulata, in which there are two muscular coats, 

 with intermediate lateral nerve-trunks. The deviations from the ordinary aspect 

 in the Zetlandic specimen may prove to be accidental. 



The elaborate system of muscles in the body- wall of these worms enables them 

 to perform the most varied and complex motions, so that they have not inaptly 

 been compared to a piece of living caoutchouc. When irritated, the larger species, 

 such as Borlasia lactea, Mont., and the true B. octoculata, suddenly contract in a 



