118 Macueay Memortat VouuMe. 
definite fibrous investment, and in being perforated by numerous bands of parenchyma 
muscle. The protoplasm has a more or less radiate structure, and has very much 
the appearance of being permeated by narrow branching channels. 
Cells similar to the excretory cells described above, but much smaller and devoid 
of canals, occur here and there, and have to be mentioned here as perhaps belonging 
to the same system. 
It will be seen that there are several important lacunz to be filled up in the 
above account of the excretory system of the Zemmnocephalee. The precise arrange- 
ment of the few “flames” that occur outside of the terminal sacs remains to be 
determined, and I am still uncertain if any of the capillaries end otherwise than by 
entering the substance of the excretory cells. Mature specimens are not favourable 
for the determination of such points, being too dense, and, in most cases, too strongly 
pigmented ; and they are only likely to be cleared up by the examination with the 
best objectives of young specimens removed from the egg. The development of the 
system, I need hardly add, must be traced before some of its relations can be 
understood. 
X.—Tue Nervous System. 
The histological elements of the nervous system are the nerve-tubes and the bipolar 
and unipolar ganglion-cells. Preparations made by a great variety of methods have 
failed to throw further light on the structure of the nerve-tubes. They are cylindrical 
tubes, varying greatly in diameter—from ‘005mm. to ‘02mm. in 7° fasczafa, with a 
firm wall of what looks like condensed fibrous tissue. In the interior is a very finely 
reticulate matter which is only acted on, and then very slightly, by colouring fluids 
after all the other tissues have become intensely stained ; from its tendency to shrink 
under the action of hardening agents this contained matter is evidently of a soft and 
more or less fluid character. Béhmig’s* account of the minute structure of the 
fo} 
nerve-tubes in the Rhabdocela, with the contained bundle of parallel fibrils does not 
apply to Zemnocephala; in the latter the fibrils, when distinguishable, appear as 
excessively fine threads which anastomose irregularly. 
The unipolar nerve-cells occur only in the brain; the bipolar frequently else- 
where in the course of the peripheral nerves. Multipolar nerve-cells such as those 
described by Béhmig were not seen except in one doubtful case, in which a ganglion 
cell which had apparently three processes was observed in the wall of the pharynx.t 
* Loc. cit. p. 257. 
+ The large multipolar cells described by Lang as occurring in 7’ristomum are, I think, the excretory cells. 
