REPORT ON THE NEMERTEA. 7 



There is, moreover, positive evidence as to the presence of a delicate nervous plexus, 

 situated just outside of the basement membrane already alluded to, and which, judging 

 from the available fragments, is present throughout the whole length of the animal. 

 The transverse sections also show that a second nervous plexus may be presumed to be 

 present just outside the inner layer of circular muscles. The details of this will be 

 discussed hereafter when considering the nervous system. 



Passing forwards along the lateral nerve-stems we find them in every section fixed 

 to the subjacent muscles by semicircular fibres indicated in figs. 7 and 8 on PI. III. 

 Anteriorly the stems pass insensibly into the brain, which, as a mere thickening of the 

 lateral stems, has as yet only attained to a very low degree of differentiation. In this 

 respect the brain much resembles that of Carinella, from which, however, it differs in a 

 very important point, viz., the presence of a posterior lobe into which penetrates a 

 ciliated duct ending blindly and communicating with the exterior. 



This posterior lobe is situated, as is the anterior one, outside the muscles of the body- 

 wall (PL VI. figs. 1-3), the inner channel is coated by a ciliated epithelium, differing in 

 texture from the surrounding nerve-cells. The latter, however, can hardly be sharply 

 distinguished from adjacent cells of a more indifferent character, and belonging to the 

 lower strata of the integument. 



The muscular elements partaking in the formation of the body- wall are kept distinctly 

 apart from the tissue, which we have described as the integument, by the homogeneous 

 membrane above mentioned. Below this membrane we find a thin, circular, muscular 

 layer (PL XL figs. 1, 2, (3), then follows the much thicker layer of longitudinal fibres (a), 

 and finally an inner layer, thinner again, of circular fibres (8). The comparative thickness 

 of these two latter layers throughout the oesophageal region may be gathered from 

 PL II. fig. 5. In the two circular layers the fibres appear to be more closely set than in 

 the longitudinal. In the outer circular layer the direction of all the fibres is, however, not 

 perpendicular to the body-axis, a very regular network of other fibres which have their 

 direction at an angle of 45° both with the longitudinal and the transverse axis, being closely 

 interwoven with this layer. These are, however, not massive enough to form a distinct 

 layer by themselves. The homogeneous intercellular substance, which is also present here 

 between the bundles of muscular fibres, and which stains very distinctly with picro- 

 carmine, is of course best visible when the bundles are widest apart. Such a portion is 

 figured in PL III. fig. 6. This intercellular substance is also seen to be again traversed 

 by radial fibres passing between the two circular layers ; nuclei are, moreover, present 

 both in the intercellular substance and enclosed along with the bundles of fibres. 



Within the muscular body-wall are lodged — (l) the proboscis and its sheath ; (2) the 

 intestine ; (3) the blood-spaces ; (4) the nephridia ; and (5) the generative sacs. The 

 space not occupied by any of these is entirely filled up by a tissue, which I will call the 

 gelatinous tissue, and regarding which more ample details will be given in the chapter 



