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



portant to the Hertwigs, 1 viz., the question whether the delicate longitudinal fibres 

 composing the longitudinal muscular layers, and which in transverse sections are often 

 arranged in rings (PI. III. fig. 6 ; PL XIII. fig. 6), have their matrix cells between them 

 or not. Having very often succeeded in demonstrating an evident nucleus in the midst 

 of this ring of cut fibres, and having constantly observed a difference in the effect of the 

 staining reagent upon this central space and upon the connective tissue surrounding the 

 muscle bundles, I must answer this question in the affirmative. 



Finally, two points deserve a short notice in this place. First, that the layer a 

 of the longitudinal Nemertean muscles is very often separated into two, a right and a left 

 half, by a dorso-median, sometimes also by an additional ventro-median solution of con- 

 tinuity. It is very marked in the primitive Palseonemertea, especially in those cases 

 where this layer is the preponderating component part of the body musculature (cf. 

 PL XL) ; it is very rarely wholly absent in Ev/polia and the Schizonemertea ; it is less 

 marked or even absent in the Hoplonemertea. In how far this separation may have 

 general morphological significance, will be discussed in the chapter devoted to general 

 considerations. 



The second point on which I shall offer speculative remarks in that chapter (cf. p. 127) 

 has again reference to the same muscular layer. Sometimes it was observed that in this 

 layer darker patches of contracted fibres alternate with lighter ones in which these 

 contractions were absent (PL XV. figs. 9, 10). As this phenomenon of contraction was 

 not wholly local but stretched all round the body in rings, the question must be considered 

 whether we have here successive waves of contraction preserved at the moment of death, 

 or whether the phenomenon has a deeper significance, is more permanent and indeed 

 allows of direct comparison with myotomes. The comparative rarity of the phenomenon 

 for the present prevents us from very emphatically advocating the latter view. 



How the circular layer of the Schizonemertea only stretches to the posterior brain-lobes, 

 how the longitudinal muscles decussate in all directions in the head, how the develop- 

 ment of the cephalic musculature is ontogenetically separated from that of the body 

 musculature, are points already known to former investigators of the anatomy and 

 embryology of the group. The fact of their having found ample discussion and mention 

 in other monographs, and the Challenger material not having furnished new points of 

 interest, will explain my silence in this Report on these and other points (such as the 

 muscular dissepiments, the musculature of the cephalic slits, &c.) connected with the 

 muscular system. 



1 Die Coelomtheorie, p. 37. 



