— 93 — 
rest wich they so well deserved and that they might now be left in 
peace in the rubric of nervous elements. 
Seeing that, comparatively, few authors have been in doubt as to 
the nervous nature of the large nerve-tubes of the Polychætes, it is really 
very strange that there has been so much dispute about these organs 
in Lumbricus. — If we examine them closely under high powers of 
the microscope, in carefuUy prepared sections, we will, instead of the 
homogeneous contents usually described in them, find a contents 
with quite the same structure as above described in the large nerve- 
tubes of Nereis, it being composed of a large bundle of primitive 
tubes, the spongioplasmic sheaths of which are, however, very thin 
and but little differentiated ; the primitive tubes are thus extremely 
difficult to observe, and this is naturally the reason why no author 
has noticed any striation in these large tubes, and why their contents 
has always been described as being homogeneous. 
I have not been able to find any spongioplasmic reticulation or 
septa, similar to what Leydig has described and illustrated (1. c. 
1886 p. 594). I cannot therefore explain their absence in any other 
way, than that his preparations have not been quite succesful, there 
may, perhaps, have been some irregularities in them produced by 
shrinking of the tube-contents, which indeed very often happens, 
especially in those thick tubes. From my own sad experience, I can 
testify that it sometimes happens in spite of an apparently very 
careful preparation. Judging from Leydigs illustration I should 
also say that such has been the case. 
The whole contents of the three nerve-tubes consist of primitive 
tubes, having the same size and diameter throughout, as will be seen 
in fig. II, and no concentration towards an axis is visible in the 
centre of them.') 
The sheaths of the nerve-tubes. — The three large nerve- 
tubes are surrounded by very thick and prominent neuroglia-sheaths 
consisting of many layers or membranes of connective substance. 
When Leydig says that they are closely surrounded by several 
slender nerve-tubes, he is scarcely quite correct. I have certainly 
observed nerve-tubes between them, and in the neuroglia surround- 
ing them, but they are scarce, and, in my opinion, the three large 
nerve-tubes are principally surrounded by connective substance, or 
neuroglia, forming thick sheaths round them. This neuroglia does 
not, however, differ from the neuroglia of the rest of the nervous 
system, in anything, else than that it occurs in thicker layers than is 
^) In my preparations the diameter of the primitive tube measured about 
.0016 Mm. 
