460 ARACHNIDA ACARINA CHAP. 



p. 470), which sometimes possess eight, some being prolonged into 

 the coxae of the legs, as in Spiders. At the sides of the anterior 

 part of the stomach there are usually two glandular bodies, the 

 pro-ventricular glands. In those Mites in \vhich the alimentary 

 canal is most differentiated (e.g. Oribatidae) three parts are dis- 

 tinguishable behind the stomach, a small intestine, a colon, and 

 a rectum, but in most groups the small intestine is practically 

 absent. The Malpighian tubes, very variable in length, enter 

 at the constriction between colon and rectum. 



In some of the Trombidiidae there appears to be a doubt as 

 to the existence of a hind-gut at all. A body having the 

 appearance of the hind-gut, and occupying its usual position, is 

 found to contain, not faecal matter, but a white excretory sub- 

 stance, and all efforts to discover any passage into it from the 

 stomach have been unsuccessful. Both Croneberg 1 and 

 Henking 2 came to the conclusion that the stomach ended 

 blindly, and that the apparent hind-gut was an excretory organ. 

 Michael, 3 in his research upon a Water-mite, Thyas petrophilus, 

 met with precisely the same difficulty, and was led to the belief 

 that what was originally hind-gut had become principally or 

 entirely an excretory organ. 



The nervous system chiefly consists of a central ganglionic 

 mass, usually transversely oval, and presenting little or no 

 indication of the parts which have coalesced in its formation. 

 Nerves proceed from it in a radiate manner, but no double nerve- 

 cord passes towards the posterior end of the body. As above 

 stated, it is perforated by the oesophagus. 



The vascular system is little understood. In 18 73 Kramer 4 

 wrote that he was able to perceive an actively pulsating heart in 

 the posterior third of the abdomen in specimens of Gamasus 

 which had recently moulted, and were therefore moderately 

 transparent. No other investigator has been equally fortunate, 

 though many capable workers have sought diligently for any 

 trace of a dorsal vessel in various Acarine groups. 



It would appear that the blood-flow in most Mites is lacunar 

 and indefinite, aided incidentally by the movements of the 

 muscles, and perhaps by a certain rhythmic motion of the 



1 Bull. Soc. Nat. de Moscow, liv. 1879, pt. i. p. 234. 



2 Z. wiss. Zool. xxx vii., 1882, p. 553. 



3 P. Z. S. } 1895, p. 174. 4 Arch. f. Naturg. i., 1876, p. 65. 



