372 



AKACHNIDA. 



Fig. 187. 



the urinary secernents of more highly organized animals, and have not 



scrupled to apply to them the appellation of renal vessels : but this hasty 



application of names we have already animadverted upon as being highly 



prejudicial to the interests of 



science ; and in this instance, 



as in many others, to wait for 



the results of future investiga- 



tions is far more advisable than 



rashly to assign a definite func- 



tion to a part the real nature 



of which is a matter of specu- 



lation. 



(961.) The respiratory sy- 

 stem of the Pulmonary Arach- 

 nidans is constructed upon very 

 peculiar principles, being neither 

 composed of gills adapted to 

 breathe water, nor of lungs like 

 those of other air-breathing 

 animals, but presenting a com- 

 bination of the characters of 

 both. Thepulmonibranchice are, 

 in fact, hollow viscera resem- 

 bling bags, the walls of which 

 are so folded and arranged in 

 laminae, that a considerable sur- 

 face is presented to the influence of oxygen. It is, indeed, highly pro- 

 bable that these organs are intermediate in function as well as in struc- 

 ture between an aquatic and air-breathing 

 respiratory apparatus ; for, as both the pedi- 

 palp 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 pulmonibranchia opens 

 externally by a distinct orifice resembling 

 the spiracle of an insect, and is closed in a 

 similar manner by moveable horny lips. In 

 the Scorpion (fig. 184) 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 re- 



markable organs (repented in the figure), 



resembling a pair of combs, which are ap- minal saccuius receiving the ter- 



., 11-11 i .-, minations of the secerning vessels 



parently adapted to keep the spiracular ( 00 ). 



Digestive and circulatory apparatus of the 

 Harvest Spider : a, the stomach, with its lateral 

 caeca, on which is situated the dorsal vessel ; b 6, 

 vascular sinuses. 



Fig. 188. 



Termination of the alimentary 



