12 ZOOLOGY. 



more. Tlie integument is more or less hard, the eyes are either sessile or 

 elevated upon movable j^edicles, 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, afibrd characters for their 

 classification. 



Class 4. Arachnida. 



This class is provided with articulate limbs, and includes spiders, mites, 

 and scorpions. They differ from insects in wanting antenna?, 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 tlie skin 

 instead. The Pulmonaria (spiders and scorpions) breathe by a kind, of 

 lungs, or pulmonobranchiffi ; the Trachearia (including the mites and some 

 small aquatic species) by tracheae, as in insects. Most of the Arachnida 

 are predaceous in their habits. 



Order 1. Pulmonaria. PI. 74, fig. 35, Amnea. 

 " 2. Trachearia. " " 36, 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 trachea?. 

 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, liaving attained their full volume previous to their final 

 transformation. 

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