390 
Tetrarliynclms erinaceus 
anterior and posterior. In the series of sections from which Text-figs. 5 
and 6 have been reconstructed there are indeed no traces of truly nervous 
tissues of any kind—neither cells nor fibres. All that is present is 
a modified parenchyma consisting of a relatively coarse and deeply 
staining reticulum, the fibres of which are arranged to form a network 
enclosing polyhedral or rounded spaces, and including verj" few nuclei 
within itself. This .tissue is very similar in nature to the modified 
parenchyma which is found beneath the basement membrane of the 
integument; it is not definitely bounded but is continuous with the 
general parenchymatous tissue which surrounds the organs of the scolex 
and proglottides ; and it is a supporting structure, or neuroglia, for the 
essential elenjents of the nervous system. When the scolex is stained 
so that the blue colouration obtained from methyl-blue-eosin becomes 
predominant, and the red and purple reaction is faint, or absent, 
this neuroglial tissue is all that one sees. 
In sections made from a scolex which has been fixed with Zenker’s 
fluid, and stained with methyl-blue-eosin so that the red and purple 
hues predominate, the appeai’ance of the tissue is quite different. The 
nervous system is now much more definitely circumscribed and one can 
make out the existence of a sheath such as is described by Pintner 
(1880)—a very delicate capsule on which are situated some scattered 
nuclei. The parenchyma cannot now be distinguished as a separate tissue 
element. Within this sheath there is—in the structures described as 
anterior and posterior ganglia, anterior commissures, bothridial and 
proboscidial nerves, and connectives—only a kind of “ neuropil ” a fine 
granular substance staining a faint red with methyl-blue-eosin, and 
evidently consisting of the sections of verj^ fine fibres cut in all planes. 
There are no nerve cells in any of the parts of the central nervous system 
mentioned above; and it is only in the posterior unpaired commissure, 
and in the short columnar ganglia situated on the courses of the lateral 
nerve cords, that true ganglion cells are to be found. These ganglion cells 
are both unipolar and multipolar; they are rather large (about 40 jjb 
in longest diameter) and have all the structure usually associated with 
such cells in other invertebrates. They are very easily seen in properly 
stained sections and I have no doubt as to their absence in all the parts 
of the nervous system except in the posterior commissure and in the 
secondary ganglia on the lateral nerve cords—the only parts of the 
whole series which can properly be regarded as nerve centres. Now 
the use of the terms “ ganglion ” and “commissure” in relation to the parts 
of the nervous system of TetrarkyncJvus appears to be of doubtful 
