ARACHNIDA. 



259 



cular walls, which action can frequently be discerned through the 

 thin integument of the smaller spiders. M. Duges, w^ho succeeded 

 in throwing a solution of carmine into the heart from behind for- 

 wards, found a plexus of vessels at the base of the pulmonary lamellae, 

 and these productions of the breathing sacs were coloured rose-red by 

 the injection of their capillaries. Hence it would appear that the 

 heart of the Arachnida served the purposes of both pulmonic and 

 systemic circulations, as Hunter discovered to be the function of the 

 heart in the flying insect. An artery is continued from both extre- 

 mities of the heart ; the anterior aorta (c) immediately gives off two 

 transverse branches (d), which Duges regards as pulmonary arteries : 

 the posterior vessel soon divides into the genital arteries (e). The 

 venous apertures are bivalvular^, as in the heart of the scorpion. The 

 heart is situated in all Arachnida, as in the other Articulata, beneath 

 the dorsal integument and above the alimentary canal. The blood 

 contains colourless round corpuscules, which have been seen to cir- 

 culate in the limbs of young spiders ; returning by a less regular 

 channel than the arterial one ; the veins of the great cavities of the 

 body are doubtless irregular and wide sinuses. 



All true Arachnidans breathe the air directly : the tracheary 

 respiratory apparatus of the mites commences by a few orifices upon 

 the abdomen : in the species, parasitic on the Hedgehog (Ixodes 

 Erinacei)y there are three stigmata ; two near the sides, and one below, 

 at the middle of the abdomen : the latter is described by Audouin as 



a spherical tubercle, pierced by a number 

 of minute holes, by which the air pene- 

 trates the tracheae. These air-tubes re- 

 semble in structure, ramification, and ex- 

 tensive diffusion, the corresponding parts 

 in insects. 



The spiders of the genera Segestria and 

 Dysdera have four stigmata, situated on 

 hi\ the under and anterior part of the abdo- 

 men : the anterior one on each side {Jig. 

 112. a) is the aperture of the pulmonary 

 sac (6); the lower orifice (c) leads to a short 

 and wide cylinder, from which radiate 

 numerous trachese (d) having the usual 

 shining surface. These tubes are united 

 together in bundles and diverge to the sur- 

 rounding parts by dissociation, not by true 

 ramification, like the tracheae of mites and 

 insects. One bundle is dispersed through- 

 s 2 



112 



A^A^/ff^ 



Respiratory organs, Segestria. 



