312 ARACHNJDA. 



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 air-breath- 

 ing respiratory apparatus ; for, as both the pedipalp and spinning 

 Arachnidans frequent moist situations, the dampness of the atmo- 

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

 circulating fluids of these creatures. Each pulmo-branchia opens 

 externally by a distinct orifice, resembling the spiracle of an in- 

 sect, and is closed in a similar manner by moveable horny lips. 

 In the scorpion (fig. 140) the spiracles are eight in number, 

 placed upon the ventral aspect of the body; and just in front of 

 the first pair of breathing-holes are two remarkable organs repre- 

 sented in the figure, resembling a pair of combs, which are appa- 

 rently adapted to keep the spiracular orifices free from dirt, and 

 thus prevent any obstructions to the free ingress and egress of the 

 air. 



In the Araneidse^ the form and arrangement of the spiracles is 

 somewhat different : according to Treviranus, there are four pairs 

 on each side of the cephalo-thorax, situated immediately above the 

 insertions of the legs ; and in addition to these there is one pair 

 constantly found on the under surface of the abdomen, and four 

 pairs of smaller apertures of less importance on its upper part. 



In order to understand the manner in which respiration takes 

 place in pulmo-branchice of the structure above described, it is 

 necessary to suppose the existence of a vascular apparatus, by 

 means of which the circulating fluid is continually spread over the 

 laminae of the respiratory sacculi, and afterwards returned to the 

 circulation in a purified condition. It is true, that, owing to the 

 extreme difficulty of tracing vessels of such small dimensions, the ' 

 continuity of the entire system is rather an inference deducible 

 from agen eral review of the facts ascertained, than absolutely a 

 matter of demonstration. We will, therefore, briefly lay before 

 the reader the data upon which physiologists found the opinions 

 entertained at the present day relative to the means whereby the 

 circulation of Arachnidans is accomplished. 



(359.) According to Treviranus, spiders are provided with a long 

 contractile vessel, which runs along the mesial line of the back, and 

 resembles in form the dorsal vessel of insects, although in struc- 

 ture it is widely different. In insects, it will be remembered, the 

 dorsal vessel communicated freely with the abdominal cavity by 

 numerous valvular apertures, and neither arteries nor veins were 



