138 ARACHNIDANS. 



principal divisions, the head, the thorax, and the abdomen ; but in 

 the spider tribes, the bloodthirsty destroyers of the insect world, 

 the separation of the head from the thorax, which, by increasing 

 the flexibility of the external framework, diminishes the strength 

 of the body, is no longer admissible. In the Arachnidans, there- 

 fore, the head and thorax are conjoined, leaving only two divisions 

 of the skeleton, the cephalo-thorax* and the abdomen. 



Insects were found, in their mature state, to have only six legs, 

 but in the adult Arachnidans eight ambulatory limbs are always 

 present. The lower forms of the Arachnidans breathe in the same 

 manner as insects, by means of air-tubes distributed throughout 

 the interior of their bodies ; but in spiders and scorpions, the most 

 typical groups, the respiratory apparatus is constructed upon very 

 peculiar principles, being neither composed of gills adapted to 

 breathe water, nor of lungs like those of many other air-breathing 

 animals, but presenting a combination of the characters of both. 

 Their breathing organs resemble bags, the sides of which are so 

 folded and plaited that a considerable surface is presented to the 

 influence of oxygen. It is, indeed, highly probable that these 

 organs are intermediate in function, as well as in structure, be- 

 tween an aquatic and an air-breathing apparatus, for as these 

 creatures always frequent moist situations, the dampness of the 

 atmosphere may be favourable to the due action of the air upon 

 their circulating fluids. To these remarkable breathing organs 

 thztevmpulmo-branchice has been applied, a name descriptive of 

 their combined function of lungs and gills. Each pnlmo-branchia 

 opens externally upon the under surface of the body by a distinct 

 orifice, resembling the spiracle of an insect, and closed in a similar 

 manner by moveable horny lips. In the scorpion (Fig. 55), the 

 spiracles are eight in number, placed upon the ventral aspect of 

 the trunk ; and just in front of the first pair are two remarkable 

 organs, represented in the figure, which resemble a pair of combs, 

 and are apparently adapted to keep the spiracular orifices free 

 from dirt, and thus prevent any obstruction to the free ingress 

 and egress of the air. 



The above characters would in themselves be sufficient to dis- 

 criminate between the two classes ; but when we add that, in the 

 Arachnidans, the eyes are always simple, and the antennae of 

 insects represented by organs of a totally different description, we 

 need not enlarge further upon the distinctions between them. 



cephale, the head; and 6upa%, thorax, breast-plate. 



