64 THE VOYAGE OF H.M.S. CHALLENGEE, 



sometimes are found to be extremely tliiu nerve-stems connecting the cellular integu- 

 ment with the central nervous system, whereas in other cases their spii-al coiling and their 

 affinity for staining solutions permits us to define them as contractile or elastic fibrils. It 

 is, however, not to these radial fibrils that the extreme pliability and continual change 

 in thickness of the basement membrane of the Hojjlonemertea can be ascribed. This 

 phenomenon must be an inherent quality of the tissue itself, and may be studied in every 

 transverse section, where the outer boundary line of the basement membrane is only 

 very rarely parallel to the inner one. Generally it is strongly undulated, in accordance 

 with the folds and wrinkles into which the integument may be thrown, not only during 

 life, but also when the animal is preserved in spirit. The consequence of this undulation 

 is, that in several places the integument much more closely approaches the muscular 

 body- wall than in others, where it is kept very widely apart, the basement membrane 

 being in the fii'st case compressed ; in the second, extended to its utmost limit. 



Together with this extension and contraction, the fine parallel stratification changes 

 its aspect, becoming more coarse, and sometimes so coarse that it might be difficult not to 

 look upon the basement layer as composed of fibres. A confusion with subjacent mus- 

 cular layers would in some cases be pardonable. A comparison of longitudinal and 

 transverse sections reveals, however, the fact that it is indeed no fibrillar, but a stratified 

 condition. Another change accompanying these phenomena of extreme plasticity is the 

 change in colour, the staining appearing far more intense when the strata are in the 

 contracted than when they are in the expanded condition. The nuclei remain visible in 

 both cases. How the change of shape and the successive expansions and contractions 

 are actually brought about in this homogeneous though laminated tissue, which has more 

 the appearance of being intercellular ground substance than anything else, must here 

 remain an open question, which we shall again meet when treating of the contractions of 

 the muscular body-wall. 



In PI. VIII. fig. 13, and PI. X. figs. 1, 2, different aspects of the Hoplonemertean 

 basement membrane are given. I will now pass to those of the Palseonemertean genus, 

 Enpolia, that in so many respects leads over to the Schizonemertea. In certain speci- 

 mens of this genus an arrangement, more or less corresponding to what has just been 

 described, was in a few cases met with, the basement membrane being of about the same 

 thickness aU round, finely striated, with imbedded nuclei, and sharply separated from the 

 integument (PI. VII. figs. 3, 9, Bet). 



In other cases the membrane separating the muscles and the integument is much 

 more folded, more irregularly striated, and less characterised as a separate band of distinct 

 tissue (PI. VII. figs. 2, 5, Bet ; PI. X. fig. 6, B). Especially in these latter cases it is 

 quite clear that this band of tissue, to which the name of basement memhrane can only 

 be applied with particular restrictions, imperceptibly passes into the connective tissue 

 found between the longitudinal muscular bundles of the outer layer, where it is moulded 



