ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
597 
from the segments which follow ; this difference may be exhibited by 
peculiarities of the alimentary canal, the circulatory system, the arrange- 
ment of the septa, the absence of nephridia, and so on. Prof. Bourne’s 
remarks and diagrams illustrative of the processes of gemmation, which 
could not be reproduced otherwise than in extenso , should be studied by 
all microscopists who have the opportunity of observing worms of this 
group. 
The author gives a system of the Naidomorpha, with definitions of 
the genera and notes on the species. 
Histology of Nervous System of Hirudinea.* — Dr. E. Eohde has 
specially investigated the minute structure of the nervous system in 
Aulostomum gulo and Pontobdella muricata. All the parts of this system 
consist of a more or less distinctly fibrillar spongioplasm, and an 
inclosed and apparently homogeneous hyaloplasm ; the former is in- 
tensely, the latter hardly at all stained by colouring matters; osmic 
acid reduces the spongioplasm, but leaves the hyaloplasm almost 
untouched. 
As to the ganglia he notes that each ganglion is divisible into a 
central substance and a peripheral layer of ganglionic cells ; this last 
consists of cells imbedded in a fibrillar supporting tissue ; these fibrils 
are the processes of supporting cells, of which there are six in each 
ganglion. Each of these six cells incloses by its processes a definite 
number of ganglionic cells ; each ganglion is consequently divided into 
six pockets which are sharply separated from one another. The spongio- 
plasm of the central substance is formed of irregularly distributed 
^ fibrils ; that of the ganglion-cells form a plexus of fibrils. The pro- 
cesses of these cells are, as a rule, finely fibrillated, and appear to pass 
gradually into the stronger fibrils of the central substance; only a 
proportionately small part pass directly into the commissures and 
nerves. The nerves contain three different elements, which are distin- 
guished by the characters of the fibrils of their spongioplasm. The chief 
part (the central substance) is the continuation of the central substance 
of the ganglion, and consists of fibrils of equal size to its. Nerve-tubes 
are differentiated from the central substance, at definite points ; these are 
ensheathed in circular fibrils. The nerves of Pontobdella contain fewer 
and thicker fibrils than those of Aulostomum. 
The spongioplasm of the commissures which connect the ganglia with 
one another is made up of fibrils of about the same size as those in the 
central substance of the nerves ; those fibrils unite at definite points to 
form thick radial septa. 
A number of the nerve-cells in the nervous system differ essentially 
in their structure from the central ganglionic cells. For example, there 
are peripheral ganglionic cells, visible to the naked eye, which are not 
unipolar but have a large number of processes, and their fibrils are much 
larger than the ordinary. 
Evidence is offered to show that the spongioplasm is only a supporting 
substance ; the identity of the fibrils of the central substance of the 
ganglia, commissures, and nerves on the one hand, and of the central 
ganglionic cells on the other, is shown by certain structural characters 
* Zool. Beitr'age, iii. (1891) pp, 1 G8 (7 pis.). 
1891. 2 U 
