56 THE VOYAGE OF H.M.S. CHALLENGER. 



the chief diflference being that the basement membrane upon which the whole integument 

 rests, is not here, at least not in all specimens, so clearly defined, nor comparatively so 

 structureless, as it is in Carinina, and as we shall again find it in the Hoplonemertea. 

 Moreover, identification is somewhat obscured hj the aj)pearance of a second, homo- 

 geneous, very thin basement membrane, which has also a strong affinity for the staining 

 reagent, and which we must be careful separately to distinguish, if we wish to establish 

 an adequate comparison between the parts in the difi"erent genera. This second basement 

 membrane (A in PL VII. figs. 2, 3, 5, 6, 9; h, in PI. XII. fig. 2 ; PL XIII. fig. 6) divides the 

 integument into two strata — an external one, comprising the peripheral sense-cells and 

 ciliated cells, the unicellular glands, and the layer of deeply-stained nuclei ; and an 

 internal one, containing the longer and more tortuous glands, the deepest integumentary 

 tissue, and, moreover, at least two very thin layers of fibres. 



If we count the thin basement membrane alluded to for one layer, this makes three 

 strata externally and four internall}^ to it — eight altogether. 



The difierent aspects of these strata may be gathered from the figures on PL VII. Fig. G 

 shows the three layers outside the membrane B, and though the histological elements 

 were not isolated, it was very obvious that large unicellular glands were here pouring 

 their secretion to the exterior. The very outermost layer was here, as in aU Nemertea, 

 formed of strictly radially arranged cells, with far less distinct nuclei, whereas between 

 and just below the secreting cells strongly stained nuclei give to this part of the integu- 

 ment the peculiar radially striped appearance which it has when viewed with lower 

 powers (figs. 2, 3, 5, 9). This same peculiarity is only less visible in fig. 6 because 

 of the very copious discharge of secretion in the gland-cells. From figs. 5 and 9 

 it is sufficiently obvious, however, that these glands are not the only ones, but that 

 in the layer indicated by Gi the darkly-stained secretion of more deeply Ipng, larger, 

 and more irregularly-shaped glands is unmistakable, and is also seen to communicate 

 with the exterior by fine tortuous tubes piercing the superposed membranous and cellular 

 strata, about ten of these ducts being specially indicated in fig. 5. Their direct passage 

 into the respective glands is not always visible in one section, the course of the tubes being 

 tortuous. The same glands, though also present in the sections shown in figs. 2 and 3, are 

 there less marked, because the secretion has not yet so distinctly accumulated. Here, too, 

 the reference letters Gi, point to the stratum in which we find them imbedded. A second 

 constituent of this stratum is seen in cells similar to the secreting cells in the unriije 

 stage (figs. 2, 3), but having afterwards a very distinctly vacuolated character, and 

 then forming the surrounding and sustaining tissue for the functional glands. They 

 might, then, best be designated as vesicular connective tissue (" blasenformiges 

 Bindegewebe"), with hardly any intercellular substance. The comparative thickness 

 to which this part of the integument may attain is best understood from PL VII. 

 figs. 5, 9, Gi. That there is a sharp line of demarcation between it and the gelatinous or 



