236 PROCEEDINGS OF SECTION D. 
parently, afford us examples of a variety of conditions ranging 
through various degrees of commensalism to true parasitism. 
Studding the stems of the branchiz in. certain crayfishes are 
species of * Pyxidium and of Lagenophrys—Infusorian genera, 
which, though probably. they occur in other situations, yet con- 
stitute an important feature of the fauna I am describing. 
Nematodes abound in the branchial cavities of most species. 
Though there are a number of different kinds, none of them 
appear to present features of any special interest. 
Various Rhabdocoeles are very constant members of this fauna. 
These are highly characteristic forms, and at least one or them 
(of which I hope to publish an account shortly) is not assignable 
to any of the recognised families. These live permanently in 
the branchial chambers, creeping among the gill filaments and 
fasten their eggs to the bases of the stems. 
Several species of Zemnocephala live habitually in the bran- 
chial cavities and lay their eggs there, attaching them to the 
epimeral plate or the branchiostegite or to the gills themselves. 
In one of these, a Tasmanian species not yet described, eyes 
are absent. Craspedella under normal circumstances never 
leaves the branchial chambers ; it occurs in this position, some- 
times in great numbers, in both Astacopsis bicarinatus and A. 
serratus. 
Actinodactylella I have only found in the branchial chambers 
of Engeus, a crayfish closely allied to the Astacopsis, but differ- 
ing from the members of the latter genus in its mode of life. 
The two known species of Engeus, E. fossor, of Gippsland, in 
Victoria, and #. cunicularius, of Western Tasmania, are never 
found in streams, but live in burrows, often of considerable depth, 
at the bottom of which sufficient moisture collects to keep them 
alive. Actinodactylella occurs in both of these species. 
Another interesting member of this remarkable fauna is a 
member of the Hzstriobdellide, a family of uncertain affinities 
commonly assigned to the Archiannelida. Hitherto the only 
known species of this family has been Histriobdella Homara, 
P. J. van Beneden (a), which is stated to live on the eggs of the 
European lobster (Astacus gammarus) (0), and is thus a marine 
and not a fresh-water animal. The new representative of the 
eroup (which I have named Stratiodrilus) lives in the bran- 
chial cavities of Tasmanian fresh-water crayfishes. It differs 
from Histriobdella Homari in certain points, one of the most 
important of these differences being the presence of paired cirri 
on the segments of the body. Stratiodrilus appears to be con- 
(a) A. Foettinger. ‘‘ Recherches sur Porganisation de Histriobdella homari. P. J. Van 
Beneden, rapportée aux Archiannélides.”’ “Arch, de Biol. v. (1884). 
(2) It is remarkable how little we know of the habits of this species. The statement 
made above is repeated bv writer after writer. But evidently it cannot be regarded as 
complete. The Histriobdella must have a more permanent abiding-place than ‘the eggs, 
which only remain for a short period. 
