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



this nervous layer tlie longitudinal muscular coat 7 is situated. There is, moreover, 

 present a third longitudinal nerve-stem, also situated, as are the two lateral ones, in this 

 plexus, but medio-dorsally in the vertical plane that passes through the animal. It is 

 this nerve which I have in a former publication (ix) proposed to call the proboscidian- 

 sheath nerve, but of which I will, in the chapter devoted to General Considerations, offer 

 ■a modified interpretation (p. 131) and which I will henceforth call the medullary nerve. 



The nerve-plexus uniting the three longitudinal nerve-stems, as a cylindrical coat of 

 tissue between the longitudinal and the circular layers of muscles, cannot be separated or 

 spread out flat, nor can we succeed in getting horizontal sections of it, just because of 

 this cylindrical curvature. A portion of it may, however, be contained in the few 

 consecutive sections passing in a horizontal plane through the medio-dorsal nerve or 

 elsewhere, tangential to the cylindrical surface of the nerve-plexus. 



From such horizontal sections figs. 2, 3, and 4 of PI. XIII. and fig. 1 of 

 PI. XIV. have been taken, and where the plexus {n. 2)1) is touched right and left of the 

 medio-dorsal nerve m, it has wholly the appearance of a dense network, the meshes of 

 which are more especially due to the fact that radial bundles of contractile tissue — by 

 which the muscular layers and the integument are held together, and which may even 

 pass from the dorsal to the ventral body-wall of the animal — pierce the nerve-plexus. 

 The longitudinal dorsal nerve stands out very boldly in the midst of the plexus. It 

 is extremely important, and may be verified in any other surface section of the nerve- 

 plexus, that from this dorsal nerve spring, both right and left, at more or less regular 

 distances, thicker tracts of nerve-tissue {tr. n.), also forming part of the plexus, but being 

 straight instead of tortuous, and having altogether the character of metamerically arranged 

 nerve-stems that are not yet recognisable as independent structures, but that are fairly 

 on the way to special differentiation as so many chief conducting tracts of nervous 

 energy in the midst of the plexiform nervous tissue which binds them together. 



The presence of these transverse stems may also be noticed in transverse and longitu- 

 dinal sections as a local thickening of the plexus, but as the whole stem is rarely attained 

 in one transverse section, this thickening may be followed in consecutive sections, and is 

 found stretching from the medio-dorsal down to the lateral nerve-stems.^ How far these 

 transverse stems may be said to be metamerically arranged, everyone may judge for 

 himself by consulting fig. 1 of PL XIV. The chief tracts are certainly symmetrical, 

 i.e., spring from the longitudinal dorsal medullary nerve at oj^posite points, and about 

 the same distance may also be seen to separate each successive pair from the foregoing. 

 Other transverse bundles, some thinner, some thicker, some more obliquely placed, &c., but 

 aU similarly forming part and portion of the plexus, are, however, visible between the 



^ Von Kennel, who has so considerably advanced our knowledge of tlie Ncmertea, appears to have observed, as early 

 as 1879 (Die in Deutschland gefundenen Landplanarien, p. 39), the presence of certain of these transverse dorsal 

 nerve-stems (commissures, v. Kenn.). He did not, however, notice or describe the nerve-plexus, nor the fact of the 

 existence of a ventral connection, both by means of the plexus and of ventral metameric stems. 



