THE ENTEEOPNEUSTA. 43 



diminution were due to the continual separating of efferent 

 fibres from the cords it would reasonably be anticipated that it 

 would be greatest in the case of those parts of the body which 

 lie behind the collar (i. e. behind the central nervous system) ; 

 for these cords have almost the whole body to supply^ but, 

 on the contrary, it is the nervous sheath of the proboscis 

 which presents the greatest concentration, and this continually 

 thickens on approaching the collar, though the proboscis is 

 conical and its base is towards the collar. This may be 

 taken to show that this sheath of nerve-fibre is afferent, and 

 is continually increasing in thickness owing to the incoming of 

 sensory fibres from the ectoderm cells lying above it. Its 

 sudden increase on the proboscis stalk is due to the sudden 

 tapering of the base. This feature is particularly well seen in 

 B. minutus. On any other hypothesis it would seem un- 

 likely that this great deposition of nerve- fibre should occur in 

 a I'egion which is generally covered up by the anterior folds of 

 the collar. 



The Central Nervous System. — The changes occurring 

 in this structure in B. Kowalevskii after its separation 

 consist in an increase in size and iu histological differentiation. 

 As the result of these changes its anterior end comes to have 

 the structure shown in fig. 60. Among the cells lining the 

 anterior end of the lumen are always some few gland-cells. 

 The cellular part of this cord is continuous, of course, with 

 the cellular part of the skin, and the fibrous part or white 

 matter, as we may call it, with the fibrous layer of the skin. 

 Behind the lumen it has the appearance shown in fig. 78, 

 The white matter does not enclose the upper part of the cord. 

 Above it are a number of pyriform cells, probably ganglionic, 

 whose tails project into the white matter. Central to these 

 the cells are more or less irregularly grouped into strands 

 enclosing spaces. The histology of this central part of the 

 cord is very difficult, and I have not been able to determine 

 how these spaces are filled. In B. minutus {v. fig. 67) they 

 are so definite as to make it certain that they are not due to 

 reagents. 



