3&4 AKACHNIDA. 



totally different description, that the sexual apertures are either situated 

 beneath the thorax, or at the base of the abdomen, and, moreover, that 

 in the greater number of Arachnidans respiration is carried on in 

 localized lungs (jpulmonibranchice), instead of by tracheae as in Insects, 

 we need not enlarge further in the present place upon the propriety of 

 ranking the Arachnida as a separate class. These animals may be 

 grouped under three principal divisions, the first of which is evidently 

 an intermediate type of organization, combining many of the characters 

 of the Insecta with the external limbs and palpi of proper Arachnida. 



(936.) The ARACHNIDA TRACHEARIA, in fact, breathe by means of 

 tracheae resembling those of Insects, which are so arranged as to convey 

 air to every part of the system ; and we may therefore suppose that 

 their circulatory apparatus, as well as their secerning organs, conform 

 more or less to the type of structure met with in the class last described. 

 The Mites (Acaridce) belong to this division, and form a very numerous 

 family, which is extensively distributed. Some are parasitic in their 

 habits, infesting the bodies of insects ; and one, the itch-insect (Acarus 

 scabiei), is found occasionally upon the human skin. Many live in 

 cheese and other provisions, where they multiply prodigiously ; and not 

 a few inhabit leaves, or are found under stones or beneath the bark of 

 trees. Some (Hydrachna) are aquatic ; but, unfortunately, in all, from 

 their extremely minute size, the investigation of their internal viscera 

 presents so many difficulties, that but little is satisfactorily known con- 

 cerning their anatomy: even the pseudo-Scorpionidce, which are of 

 larger growth and, although still breathing by trachea}, approximate 

 most closely to the outward form of the next group, have been very 

 imperfectly examined. 



(937.) In the Acaridans, the most remarkable feature of their 

 structure is the complete consolidation or coalescence of the principal 

 divisions of the body, which are always more or less distinct in the other 

 Articulata ; for not only do we find in them the head consolidated with 

 the thoracic portion, but the abdomen likewise is swallowed up, as it 

 were, in the general covering of the body. The legs, as in other Arach- 

 nidans, are eight in number, and are generally composed of seven articu- 

 lations, of which the first, which is sometimes adherent and sometimes 

 free, corresponds with the coxa of Insects, the second with the tro- 

 chanter ; the third, representing the femur, is often more developed than 

 the rest, whilst the remaining constitute the tibia and the tarsal joints. 

 The last segment of the tarsus, or foot as it might be called, is furnished 

 with two moveable hooks, that can be folded back, and lodged in a slight 

 excavation provided for the purpose. 



(938.) In accordance with their structure, which is adapted to the 

 habits of the various races, the feet of the Acaridans may be divided 

 into such as are adapted for feeling (palpatorii}, in which the ultimate 

 joint is dilated ; for walking (gressorii) ; for swimming (remigantes), 



