J. Johnstone 
391 
propriety, but I am unable to describe them otherwise in few words so as 
to make their topography clear. Until the whole system of organs 
shall have been described in the light of information gained from a 
variety of staining and fixing methods the above terms may suffice. 
The most accurate account of the central nervous system of a Tetra- 
rhynchid in the literature is that of Pintner (1881) with reference to T. 
longicollis van Beneden. In a later paper Pintner also describes the 
nervous system of T. smaridum Pintner. This later paper is evidently 
only a preliminary description but the relationships of the nervous system 
are essentially those which I have just described with reference to 
T. erinaceus. Pintner’s PI. Ill, figs. 33 to 47 represent a series of 
sections through a -scolex and one can easily distinguish the four ganglia, 
the two paired anterior, and the posterior unpaired commissures just as 
they are represented in the present paper. There is little doubt that 
the conditions in T. smaridum are identical with, or are very similar to, 
those of T. erinaceus. In the earlier paper quoted Pintner gives a much 
more detailed description of the histology of the nervous system of 
T. longicollis and there is evidence everywhere in this fine memoir of 
accurate and close observation. The anterior ganglia are represented as 
fibrous in structure and as entirely devoid of nerve cells; and this is also 
the structure of the two anterior commissures; but instead of an 
unpaired posterior commissure Pintner describes a ganglionic mass 
occupying the same position. The whole bi’ain is described as being 
surrounded by a sheath, finely fibrous in structure and containing nuclei. 
It is described in general terms as consisting of a ganglionic centre 
surrounded by a ring-shaped commissure. 
In Tetrarfiynchus erinaceus this fine structureless membrane, or brain 
sheath, can just be seen in relation to the anterior ganglia; it does not 
appear to be present, or it is very obscure in relation to the commissures, 
while it appears to be certainly absent from the periphery of the 
posterior commissure. In a few sections made from an imperfect scolex 
fixed in vom Bath’s fluid and stained with iron haematoxylin this 
posterior commissure is pi’esent and I am sure that it contains fibres 
running longitudinally through it as well as nerve cells, which appear 
to be multipolar. But the posterior commissure is not nearly so 
compact as either of the others, and the ganglion cells are surprisingly 
few in number: in a complete series of sections they could easily be 
counted. Thus the nervous system differs from that of most inverte¬ 
brates (1) in that the periphery is fibrous while the central region is 
ganglionic ; (2) in the massive development of the commissural tracts in 
