- 97 — 
some very slender tubes inside which no subdivision is visible, and 
these we must supose to be the primitive tubes, a small bundle of 
which generally forms a nerve-tube. 
Fig. 21, which represents the transverse section of an anterior 
nerve of Phallusia venosa, will, I hope, give easily, at a glance, a 
correct idea of the structure of the peripheral nerves of the Ascidians.') 
Fig. 22 represents a part of a longitudinal section through a nerve, 
seen under high powers of the microscope; the primitive tubes are, 
here, visible, being transsected in their curved course, the curvature 
in which arises from the contraction of the nerve, ts, ts are the 
sheaths of the nerve-tubes. Some of them (a and 6) are stouter 
and more prominent than the others , those are probably transsected 
neuroglia-septa enclosing bundles of nerve-tubes. In one of these 
septa is seen a neuroglia-nucleus [n). 
The division of the nerve-tubes into different smaller and larger 
bundles is, in the Ascidians, as well as in the Molluscs, Crustaceans 
etc. less prominent near the origin of the nerv^es; close to the nerve- 
roots there is, in the Ascidians, scarcely any division visible, and a 
great many of the nerve-tubes appear, even, to be broken up into 
nerve-tubes of a much smaller diameter ; the reason of this appearance 
we will in a later chapter have an opportunity of discussing. 
The structure of the nerves of the Ascidians reminds much of 
what we have found in the Molluscs. The nerve-tubes are separated 
by only very thin sheaths (cmfr fig. 22, ts) in which thickish hbres 
are situated, especially along the concreting longitudinal edges; in 
transverse sections these fibres, when transsected, appear as darker 
dots, situated in the corners of the meshes, and are produced by 
the transsected sheaths of the nerve-tubes (vide fig. 2i). 
Su mm a ry. 
The results of, these, our researches on the minute structure of 
the nerve-tubes of various invertebrates, we may, I think, assume to 
be applicable to the nerve-tubes of all invertebrates of such a high 
organisation as to posess a well developed nervous system, i. e. all 
invertehrated hilaterates. From Lang's memoirs, we may gather 
that the Polyclades and Trematodes have nerve-tubes of the same 
type described in this chapter (cmfr. p. 52 — 54); from Fraiponts 
) The primitive tubes are not seen in this illustration. 
7 
