12 ZOOLOGY. 
more. The integument is more or less hard, the eyes are either sessile or 
elevated upon movable pedicles, as in the two highest orders. The 
situation and form of the gills, the position of the head and tail, the structure 
and number of the feet and parts of the mouth, afford characters for their 
classification. 
Orper 1. TRILOBITA. 
“62. Poscitopopa. Pl. 14, Fg. 34, Caligus (parasitic). 
‘*¢ 3. BRrancHiopopa. : 33, Cyclops (waterflea). 
«¢ 4, Isopopa. i *¢ 32, Oniscus. 
¢¢ «5, Lzmopreopa. =: rs 3t, Cyamus edaeacath 
“© 6. AMPHIPODA. E “¢ 30, Taletrus. 
«© 7, STOMATOPODA. er 29, Squilla: 
¢ 68. Drcapopa. ye “6 28, Cancer. 
Class 4. Arachnida. 
This class is provided with articulate limbs, and includes spiders, mites, 
and scorpions. ' They differ from insects in wanting antenne, in having 
simple eyes, and in having the head coalescing with the thorax, and forming 
the cephalothorax. The feet are generally eight in number, being but six 
in insects; they are not subject to a metamorphosis, but moult the skin 
instead. The Pulmonaria (spiders and scorpions) breathe by a kind of 
lungs, or pulmonobranchiz ; the Trachearia (including the mites and some 
small aquatic species) by tracheze, as in insects. Most of the Arachnida 
are predaceous in their habits. 
Orper 1. Putmonarta. Pl. 74, jig. 35, Aranea. 
(¢ 2. 'TRACHEARIA. i “386, Chelifer. 
Class 5. Insecta. 
Insects have articulated feet, a dorsal vessel instead of a heart, and they 
breathe through lateral spiracles, connected with two principal trachee. 
Some insects are apterous and some winged, the number of wings being 
either two or four. The four wings are of a similar texture in some, and 
of a different texture in others. In the Coleoptera the posterior pair alone 
are used in flight, the anterior pair being converted into covers for their 
protection whilst at rest. The number of feet is six, except in the 
vermiform centipedes and millipedes (Myriapoda), which have characters 
intermediate to the true insects and the annelida; and, indeed, they are 
regarded by some as a distinct class. Insects undergo a more or less 
complete metamorphosis, which, in the Myriapoda, is confined to an 
increase of the number of segments and feet. Insects do not grow in this 
perfect state, having attained their full volume previous to their final 
transformation. 
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