122 _ Report of the Microscopical Section, [Sess. — 
REPORT OF THE MICROSCOPICAL SECTION. 
By Mr JAMES RUSSELL, Convener. 
THE Crustacea formed the subject of study of the section 
during this session, and the type chosen was the Crayfish 
(Astacus fluviatilis). The text-books referred to were — 
Huxley’s ‘The Crayfish’ and Huxley’s ‘ Practical. Biology, 
by Howes and Scott. The course followed was to study 
the general characters and different organs in detail, and 
for this purpose a supply of crayfish was kindly obtained 
for the section by Dr Davies. 
The Crustacea form a large and important class of animals, 
ranked under the sub-kingdom Articulata, better called Annu- 
losa (ringed animals). The crayfish, the type studied, consists 
of twenty rings or somites, to each of which appendages are 
attached. The somites are divided into three sections—the 
head, the thorax, and the abdomen. The head and thorax are 
joined together in one solid mass, termed the cephalo-thorax, 
covered by a strong hard shield called the carapace; the abdomen 
is jointed or hinged to the thorax, and each of its somites is 
hinged to the one immediately in front, so that the whole of 
the abdomen can be bent inwards upon itself. The back of 
the crayfish is termed the tergum, and the under side the 
sternum, and the part which unites the two the pleuron, or, in 
the plural, pleura; while the whole of the outer covering is 
called the exoskeleton. 
Of the somites, into which the exoskeleton is divided, six 
belong to the region of the head, eight to the region of the 
thorax, and six to the abdominal regions, The first somite 
of the head carries the eyestalks, on the ends of which are 
situate the compound eyes; the second somite bears the 
antennules, or lesser antenne, on the upper side of the basal 
joint of which are situate the auditory organs, the opening to 
which is protected by a tuft of delicate seta. These anten- 
nules have each two branches, which bear fine setze; but on 
the outer branch there are, in addition to these sets, several 
bundles of curious appendages, which are thought by some to 
have olfactory functions. The third somite bears the greater — 
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