Systematic Course, 9 
undivided mantle which usually secretes a dorsal shell. 
They include all the land and water snails which have a 
shell developed, and the slugs where the shell is rudi- 
mentary. For details of structure, see the chapter on the 
dissection of the snail. The Museum contains numerous 
Gastropoda shells for the students to examine. 
(ec) Cephalopoda.—These are sea creatures, with distinct head 
and a number of sucker-hearing arms around the mouth. 
They inelude all the Cuttle-fishes, Squids, and Belemnites 
which are without external shells, and the Nautilus and 
Ammonite, where a specialised many-chambered shell igs 
developed. The group is of very little importance from a 
forester’s point of view, but the students should notice the 
Nautilus shells and the Ammonite and Belemnite fossils 
in the Museum and sketch a typical representative of each. 
(4)—ARTHROPODA. 
The Arthropoda are a vast group of bilaterally segmented Ccelomata, 
which are characterised by the possession of jointed appendages supported 
by external armour. For accounts of the structure of typical represent- 
atives, see the chapters on the dissection of the fresh-water crab, the 
cockroach and the mulberry silk-worm. The chief groups comprised by 
the Arthropoda are the Crustacea, the Myriapoda, the Arachnida, and the 
Tnsecta, 
(a) Crustacea.—The Crustacea comprise a large assortment of 
aquatic Arthropoda, which breathe by means of branchiax. 
They include lobsters, crayfish, shrimps, prawns, hermit 
crabs, shore crabs, fresh-water crabs, king crabs, barnacles, 
- water-fleas, and a number of less well-known forms. From 
the forester’s point of view they are of no very great 
importance, and the students will learn all that is requisite 
about their structure from the dissection of the fresh-water 
crab, Attention should also be given to the represent- 
atives of the group in the Schoo] Museum, sketches being 
made of a few typical forms. 2 
(2) Myriapoda.—The Myriapoda are a small group of air-breath- 
ing Arthropoda, which are characterised by having a large 
_series of legs on either side of the body. They are divided 
into the Centipedes (Chilopoda) and the Millipedes (Chilog- 
natha). The Centipedes have but one pair of legs attached 
to each segment of the body and are carnivorous in their 
habits, They are furnished-with a large pair of poison 
claws, which are modifications of the second pair of 
thoracic appendages. The Millipedes have two pairs of 
