REPORT ON THE NEMERTEA. 67 



species that, besides the distinctly granular cells included, in it, striation in nearly every 

 direction forms a prominent feature of this intermuscular substance. 



Inside the innermost muscular coat the gelatinous tissue appears in the third modifica- 

 tion which we have established for it. This modification is in many respects the most im- 

 portant, and at the same time most liable to a considerable amount of variation according to 

 the species we happen to examine. It has been already noticed that of all Nemertea Pelago- 

 nemertes shows it in its fullest development, whereas the eminently muscular Carinina is 

 only very sparsely provided with it. It will thus be safest first of all to examine it in 

 Pelagonemertes, and to indicate by what features the other genera differ and gradually 

 lead to grades of development as far down as Carinina. 



The most striking feature in examining any section of Pelagonemertes, either with 

 low or high powers, with a special view to the gelatinous tissue inside the muscular body- 

 wall, is the homogeneity of this tissue, the comparative scarceness of nuclei, and the 

 uniform imbibition of the staining reagent, by which it has obtained a delicate rose colour. 

 A comparison with the medusoid jelly, or, more distantly, with the intercellular substance 

 of cartilage, is here inevitable. 



This general feature being established, the secondary characteristics are — (1) the 

 imbedded nuclei of this jelly; (2) the fibres forming part of its substance, other fibres tra- 

 versing it in apparently tubiform channels; (3) differences in the coloration of the jelly in 

 different regions, and lastly, a fibrillation of the utmost tenuity, only visible by the aid 

 of very high powers, which appears in different regions of the apparently homogeneous 

 jelly, other and large portions, however, retaining the homogeneous aspect even with 

 these high powers, though then not appearing wholly limpid but cloudy, perhaps an indi- 

 cation of a yet finer fibrillation escaping the analysing power of our objectives when 

 studied, not in the fresh state, but in Canada balsam. 



The different coloration of the jelly in different regions is partly arbitrary, i.e., darker- 

 coloured patches are irregularly scattered throughout the general lighter hue. At certain 

 places the darker staining is, however, constant, viz., contiguous to those regions where 

 the jelly is interrupted. Thus the channels above alluded to, in which nerve-fibres and 

 others take their course through the jelly, are marked by a double boundary line of 

 darker colour (PI. VIII. fig. 6, n"), corresponding to the tract along which the continuity 

 of the jelly is interrupted for the passage of these fibres. These nervous tracts being 

 exceedingly numerous, the transversely or obliquely cut ends of similar distinctly red tubes 

 are discovered in every section (PL VIII. fig. 3). Moreover, all round the two principal 

 nerve-stems (figs. 6, 8), and bounding the cavity of the proboscidian sheath (PI. VIII. 

 fig. 12, B), the blood spaces (fig. 8, bl), and the cavities in which the generative products 

 are lodged (fig. 8), the same continuous dark red tint, which that portion of the jelly has 

 acquired by the picrocarmine, is observed. 



A peculiarity which I have further to notice in the sections, is the different hue that 



