460 ARACHNIDA—ACARINA CHAP. 
p- £70), 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 which 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! and 
Henking*® came to the conclusion that the stomach ended 
blindly, and that the apparent hind-gut was an excretory organ. 
Michael,’ in his research upon a Water-mite, Zhyas 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 1876 Kramer * 
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 
Bull. Soc. Nat. de Moscow, liv. 1879, pt. i. p. 234. 
Z. wiss. Zool. Xxxvii., 1882, p. 553. 
P. Z. S., 1895, p. 174. 4 Arch. f. Natwrg. i., 1876, p. 65. 
