AEACHNIDA. 185 



thirsty destroyers of the insect world, the separation 

 of the head from the thorax, which, by increasing 

 the flexibility of the external frame-work, diminishes 

 the strength of the body, is no longer admissible. 

 In the Arachnidans, therefore, 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, eigJit am- 

 bulatory limbs are always present. The lower forms 

 of the Arachnidans breathe in the same manner as 

 insects, by means of air-tubes distributed through- 

 out the interior of their bodies ; but in spiders and 

 scorpions, the most typical groups, the respiratory 

 apparatus is constructed upon very peculiar princi- 

 ples, 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, between an aquatic and an air- 

 breathing apparatus, for as these creatures always 

 frequent moist situations, the dampness of the atmo- 

 sphere may be favourable to the due action of the 

 air upon their circulating fluids. To these remark- 

 able breathing organs the term pulmo-hranclda has 

 been applied, a name descriptive of their combined 

 function of lungs and gills. Each pulmo-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 body ; 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 



* KecpaX}], cephale, the head ; and Qwpa^, thorax, hreast-plate. 



