438 
toR. A. A. W. IIUBRECIIT. 
That a tendency to a metameric arrangement of the other- 
wise indifferent layer is nevertheless already present in the 
ScHizoNEMERTiNi is shown in fig. 13, which illustrates a 
thin section through the longitudinal muscular layers, im- 
mediately above the nervous sheath. Three of the peripheral 
processes taking their course through interstices between 
the^ muscles have been cut in this section ; they are 
regularly arranged along a transverse row, and numerous 
other sections show that indeed an approach to regularity 
may in this respect be noticed all over the body. If this 
regular arrangement gradually combines with a thickening 
of the nervous layer along the points from whence these 
processes originate, and with a corresponding reduction along 
the intermediate spaces, the formation of transverse indi- 
vidual nerves (which at first would have the appearance 
of commissures,^ and in a later stage would take the aspect 
of transverse branches springing from the longitudinal 
trunks), carrying perpendicular smaller twigs, would be the 
inevitable result. And it is in this way that we must 
form our conceptions of the intermediate stages lying 
between Carinella and the Hoplois^emertini. As such must 
also be regarded the plexus-like arrangement between two 
successive peripheral branches, as observed by Macintosh 
and noted above. 
A certain variability in the aspect of the nervous tunic in 
small and large specimens, which, though generally a con- 
tinuous layer (even if not of considerable thickness), on 
other occasions has the appearance of a plexus with very 
narrow meshes, confirms the supposition that local varia- 
tions of the thickness of this layer, finally leading to the 
formation of separate nerve-stems, may occur. Generally 
bundles of radial fibres (^.), which in other places are more 
isolated, take their course through these meshes.‘^ 
^ In a recent paper of von Kennel’s (‘ Die in Deutschland gefundenen 
Land-planarien,’ Wurzburg, 1879, p. 39) he speaks (quite aphoristically) 
of transverse commissures uniting the longitudinal trunks of Nemerteans 
across the back of the animal. It may be that what he took for commis- 
sures are portions of the nervous sheath above described ; still it appears 
improbable that this excellent observer would have overlooked the nervous 
tissue which unites the longitudinal trunks ventrally as well. Surface 
sections (PI. XXXIII, fig. 12) show that this questionable arrangement of 
the tissue in transverse commissures, now and then met with in longitudinal 
sections, is merely apparent. 
2 When I showed some of my preparations to Prof. E. Ray Lankester 
last April, he drew my attention to the apparent agreement between this 
nervous layer and Strieker’s nerve-layer of the epiblast of the embryo- 
frog. I am not disposed to believe that any deep significance can be 
attached to this mthcr distant resemblance; but the parallelism, such 
as it is, is interesting. 
