ARACHKIDA. 



453 



side of the ^*'"*"*%^''' ^P*'*"- 



from the upper part of the stomach. The intestine is contracted 

 where it passes through the pedicle of the abdo- i66 

 men ; it slightly expands in its straight course {/) 

 along the anterior part of that cavity, then con- 

 tracts and forms two short convolutions (^), and 

 communicates with a large globular caecum (A), 

 from wliich the short rectum passes to the vent. , 

 In some mites, e. g. Ixodes, the salivary glands 

 consist of masses of vesicles situated on the sides 

 of the fore part of the body ; and pour the secre- 

 tion by branched canals into the mouth at the base 

 of the labium. In the spiders a slit in the upper 

 lip leads into a cavity at the base of which is a 

 transparent glandular mass, the secretion of which 

 flows through the slit, and moistens the substances 

 from which the nutriment is being extracted. Four 

 biliary ducts (^ i) open into each 

 straight portion of the intestine. Two longer and 

 more slender urinary tubes (A, k) communicate with the beginning 

 of the caecum, which seems to stand to them in the relation of an 

 urinary bladder. Large masses of adipose epiploon occupy, in well- 

 fed spiders, the sides of the abdomen, and cover and conceal tlie gra- 

 nular brownish caecal terminations of the voluminous hepatic organ. 



The gastric caeca and the stores of fat may both contribute to the 

 power of endurance of the prolonged^ fasts for which spiders are 

 remarkable. Mr. Blackwall kept a young Theridion ^-pimctatum 

 alive without food from the loth October, 1829, to the 30th April, 1831. 

 It had alvine evacuations at distant intervals and in small quantities 

 to the end of its existence. It also spun several snares, which were 

 successively removed by the experimentor. 



The chyle is received immediately by the veins, and conveyed to 

 the dorsal vasiform heart. The heart is situated in all Arachnids, 

 as in the other Articulata, beneath the dorsal integument and above 

 the alimentary canal. In the Phalangid<B it is three-chambered • : 

 in the scorpions it is confined to the six dilated anterior segments of 

 the abdomen, where it is of uniform diameter, except at its two 

 attenuated extremities : it receives the venous blood from the surround- 

 ing pericardial sinus by ten or eleven pairs of apertures, each guarded 

 by a pair of valves. From the anterior and larger extremity the anterior 

 aorta is continued, which is short, and soon divides into three branches : 

 a longer and more slender vessel is continued along the terminal 

 segments of the abdomen from the narrower posterior end of the heart. 



♦ CCLXV. p. 154. 

 CG 3 



