446 



LECTURE XIX. 



Before entering npon the anatomy of the typical forms of the 

 Arachnida, a few remarks may be premised on another low-organised 

 member of the class, which, on account of its remarkable power of 

 retaining life, and reviving after some years' complete desiccation, has 

 received the generic name of Macrohiotus. This minute and peculiarly 

 shaped mite, in which the hinder rudimental legs come off from the 

 abdomen, was discovered by Eichhorn in 1767 *, and was described by 

 him under the name of " water-bear " (wasser-baer). Corti, in 1774 f, 

 recounted its power of returning to life after being dried. It is not 

 uncommonly found in the gutters of the roofs of continental houses ; 

 it crawls along the sediment like a tortoise, and was grouped by 

 SpahtnzaniJ with the Rotifers, under the name of "tardigrade." 

 OthoFred. Miiller§ first detected its true relationship with the mites. 

 It is subject to many moults and oviposits in its exuviae, which, as 

 the integument is chitinous, is long preserved from decay. The 

 transparency of the skin is such, in the living animal, as to show a 

 complicated muscular system beneath, the ultimate fibre of which is 

 smooth. The nervous system consists of four ganglia, corresponding 

 with the four segments of the body ; there is no supera3Sophageal 

 ganglion. The mouth is suctorial, situated at the end of a retractile 

 proboscis, on each side of which are two tooth-like styles, — the rudi- 

 ments of lateral jaws. The stomach is oblong, occupying a large 

 portion of the body, and is divided by numerous constrictions into 

 irregularly disposed caeca ; their walls are provided with hepatic 

 cells. Near the mouth are two large lobulated 163 

 glands, that seem to be salivary. No traces of 

 respiratory organs have yet been found ; and the 

 respiration in Macrohiotus, as in Demodex, 

 must be cutaneous. 



In some of the small and parasitic tracheary 

 Arachnids or mites, certain pairs of legs are 

 terminated by adhesive suckers, and others are 

 occasionally terminated by setae, as in the itch- 

 mite (Sarcoptes Galei,Jig. 163.). 



The mouth, in all Arachnids, is situated on 

 the anterior segment, and is provided with in- 

 struments adapted either for suction or masti- 

 cation. In the parasitic mites the rudiments of 

 the jaws are more or less enveloped in a sheath formed by the lower 

 lip : the maxillary palpi are usually the only parts which have free 



Sarcoptes Galei, or itch- 

 insect, magnitied. 



* CCLXXXII. 

 X XLIII. 



t CCLXXXIII. 

 § XLIV. 



