MITES IOIJ 



hairs had an auditory function. In the Ixodidw a singular drum- 

 like structure in the first leg has been considered by Haller and 

 others to be the hearing organ ; while in the Oribatidce that organ 

 appears to be located in the pseudo-stigmata, two paired organs at 

 the side of the cephalo-thorax which were long taken for true stig- 

 niata. The Gama&idce, Oribatidce, Tyrogl>/plii<l<i', Sarcoptidce, A:c. 

 are entirely without special organs of vision. The Hydrachnidce 

 have two pairs of simple eyes, each pair being so close together as 

 to look like a single eye. The Trombidiidce mostly have simple 

 eyes, the number and position of which vary with the species. 

 As to internal anatomy it should be noted that there is almost 

 endless variety. The alimentary canal most commonly consists of a 

 long thin (esophagus, provided with distensor muscles on each side, 

 so as to make it a sucking organ ; it usually passes right through or 

 close under the great ganglion known as the brain; in some species, 

 as ])<inn< a* i/i'iiif/ifiiftis, the oesophagus is followed by a large pro- 

 ventriculus, but this is not usual ; it more commonly leads directly 

 into the ventriculus, which generally is a principal viscus, and in 

 most families furnished with more or less glandular ctecal ap- 

 pendages, not numerous, but often very large, occasionally larger 

 than the organ itself. A valve in many cases separates the ven- 

 triculus from the hind-gut, which is commonly divided into what 

 may be called colon and rectum. In the Gamasula; a single very 

 large Malpighian vessel on each side of the body enters between 

 the two last-named divisions of the alimentary canal. These vessels 

 run right along the side of the body, and strong pulsation may be 

 seen in them. In the Oribatidce they are absent, their function 

 being apparently performed by supercoxal glands. The Tyro- 

 glyphidce, Savcoplulce, Phytoptidfr, Arc. are without special respira- 

 tory organs ; the Oribatidce and some r i-ii<nTtt have simple uii- 

 branched trachea 3 , much in the same condition as those of r< j ri/>ut</x. 

 The other Gainasidw, the Trombidiidce, Cheyletidce, Ixodidtf. AT. 

 usually have branched tracheae, like insects ; air-sacs are occasion- 

 ally found, but not anything like the tracheal lungs or gills (so 

 called) of spiders and scorpions. The principal nerve-centre is 

 much concentrated, and consists usually of either a large Mipra- 

 cesophageal and smaller suboesophageal ganglion joined by com- 

 missures; or, more frequently, the whole forms one mass which is 

 pierced by the oesophagus, which may be pulled out. leaving a neat 

 round hole ; the nerves, of course, radiate from this mass, but there 

 is not space here to describe their course. A pulsating organ of the 

 nature of the dorsal vessel of insects, but much shorter, and with 

 only one or two pairs of ostia, has been detected in some Gamasidce, 

 and in I.rodes, first by Kramer and afterwards by "Winkler and 

 Claus ; it has a median aorta running forward; it is best seen in 

 life in young specimens still transparent ; it lies at the rear of the 

 ventriculus, near the dorsal surface. Nothing of the nature of a 

 heart has yet been discovered in other Acarina. The reproductive 

 organs are, perhaps, most frequently of the ' ring ' type, well known 

 in the Arachnida; thus in female Or ibatidce they consist of a central 

 ovary, with an oviduct springing from near each end, in which the 



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