REPORT ON THE NEMERTEA. 103 



said to represent a plexus with numerous longitudinal thickened portions or stems. 

 The passage of the nerves from the brain into the proboscis can very rarely be well 

 observed, because the proboscis is nearly always extruded and torn off when the 

 animal is killed. I may, however, repeat what was noticed above (p. 85), viz., that in 

 Amphiporus moseleyi, more particularly, this doubt has now been dispelled. I can 

 observe in my sections that, instead of two strong nerves innervating the proboscis, as 

 in the Palaeonemertea and Schizonemertea, a much larger number of branches leave 

 the brain-ring and enter the proboscis in the region of its attachment. That these may 

 dichotomise and give rise to a larger number of longitudinal stems has been already 

 stated by von Kennel (XVI). 



This nervous plexus and the longitudinal stems subdivide the longitudinal muscle- 

 layer into an outer and an inner portion, the latter (when the proboscis is everted) being 

 again subdivided into as many longitudinal columns as there are nerve-stems in the 

 proboscis (PI. XII. fig. 6). Outside and inside of this longitudinal layer there is a 

 circular layer of fibres, outside of the exterior one of these the epithelium. 



As to the nerve-stems and the plexus, one specimen of Amphiporus marioni showed 

 very distinct cellular accumulations just between each nerve-stem, as if a longitudinal 

 tract of nerve-cells alternated with one of nerve-fibres in the plexus. For the study of this 

 phenomenon fresh specimens will be absolutely necessary. The phenomenon itself has 

 been already noticed, but has not yet been wholly understood, either by von Kennel (XVI) 

 or by Graff (ill). 



Just as it has been necessary to curtail our observations on the proboscis because of 

 the detailed information already available concerning this important organ, the probos- 

 cidian sheath need not be treated at any length in view of the data that are already furnished 

 by others. It is known to be a closed space surrounding the proboscis, having in the 

 majority of cases its own muscular wall, by the contractions of which the fluid contained 

 in the space is driven against the anterior proboscidian attachment. The muscular 

 sheath thus serves to protrude the proboscis as far as the length of the posterior portion 

 — acting as a retractor-muscle — will allow it. 



There can hardly be any doubt, when we take into consideration all the morpholo- 

 gical data at our disposal, that the muscles composing the proboscidian sheath gradually 

 took their origin by the increase and modification of pre-existing muscular elements, 

 which belonged to the body-wall and to the body-parenchyma before the proboscis, 

 modified from a tactile organ, as it appears to have primitively been, had yet become 

 evolved, through the growth inwards of the anterior tip of the body, into an aggressive 

 weapon, with stylet or nematocysts, &c. We find the shorter proboscides, and the less 

 significant proboscidian sheaths among the more primitive genera of Nemertea. 



Carinella has a short proboscis ; the dorsal wall of its sheath is still a component part 

 of the musculature of the body-wall ; the ventral wall is thin, and only composed of a 



