ACARINA. 



845 



which pierces the concentrated nerve ganglion and then dilates into 

 a globular stomach. Salivary glands are conspicuous in some of 

 the ticks (Fig. 556). The intestine gives off a comparatively small 

 number of caeca, two of which are large and pass backward (Fig. 

 556 D) . In some cases, as in spiders, the caeca are continued into 

 the coxae of the legs. The intestine may be divided into a small 

 intestine and colon and rectum, and in any case the malpighian 

 tubules arise between the last named regions. No connexion 

 has yet been found between the stomach and the rectum in 

 certain of the Trombidiidae. In this family the former ends 

 blindly and the latter is a simple sac packed apparently 

 with excretory products. In some genera paired coxal glands 

 supplement the malpighian tubules. 



Many years ago a two-chambered heart was described in 

 certain of the Gamasidae, 

 but since that time no one 

 has succeeded in seeing it 

 and it seems probable that 

 there is no specialized organ 

 for propelling the blood in 

 mites and that the circula- 

 tion is lacunar and pro- 

 moted by the movement of 

 muscles and certain rhyth- 

 mic pulsations of the ali- 

 mentary canal. 



The respiratory organs are 

 tracheae, often long and con- 

 voluted. In aquatic species 

 they are often absent. The 

 stigmata are either paired 

 and open near the base of 

 the chelicerae, or above 

 and behind the base of the 4th, 5th or 6th appendages, or in 

 the articular sockets of these or of the 3rd pair, or finally they 

 are -united and open on the dorsal surface. This diversity 

 of position is used in classification. 



The nervous system is very concentrated ; an oval mass with 

 little indication of its component parts is pierced by the oeso- 

 phagus. From it radiate many nerves, but there is no dominant 



D 



FIG. 556. Anatomy of Ixodes ricinus (after Al. 

 Pagenstecher). G Brain; SpD salivary 

 gland ; Dg ducts of salivary gland ; D diver- 

 ticula of intestine ; A anus ; N malpighian 

 tube ; Tr bundles of tracheae : St stigma. 



