Abstract: The essential distribution model of 18 pairs of hydrachnelles' glandularia, including 2 pairs of antennal glandularia (A₁, A₂), 4 pairs of dorsoglandularia (D₁eD₄), 4 pairs of lateroglandularia (L₁-L₂), 4 pairs of epimeroglandularia (E₁-E₄), and 4 pairs of venteroglandularia (V₁-V₄), and 2 pairs of non-glandularia seta named ocularia (O₁, O₂) is defined through comparative studies on more than 120 Chinese species of water mites. It is considered as a primitive distribution scheme from whice the patterns of the glandularia for different families of water mites can be drawn. According to the structural features of the glandularial glands and sclerites, the glandularia are grouped into four distincti types, eylaoides type-the most primitive one, hydryphantoides type, hydrachnoides type and lebertioides type. The lebertioides type ispresent in three higher groups including Lebertioidea, Hygrobatoidea and Arrenuroidea, and is considered to be recent in the evolutionary scale. A new hypothesis, "eighteen-segment theory," of the origin of the mite descending from worm-like ancestor with 18 segments ( I - XVt) and a precheliceral lobe (pl), is proposed through a careful study of the glandularia. The gland structure of hydrachnelles' glandularia shows no similarity to other glandular structures or organs found in all acarides groups. This means that the 18 pairs of glandularia independently derived from 18 intersegmental folds between primary segments of the worm-like ancestor in the course of evolutionof soma with reducing body axis. The "eighteen-segment theory" presumes that the transferring of the ancestor's segments, to reduce body axis, is directed by two stable points, "das 1" between IV and v and "das 2' at area of genital pore. Toward "das 1" and "das2", I , II and pl transfer dorsally and over III and IV , VII-X dorso-anteriorly and over Vand VI , XV - XVIII ventrally and behind the genital pore. The impressive explanation of the theory is given with the terms used by Grand jean. Other theories, "sixteen-segmenttheory" by Grand jean in 1969 and Coineau in 1972 and "twelve-segment theory" by Badern 1982, and phylogenetic points of view on arthropod groups are discussed in detail.

Key words: Hydrachnellae, morphology, mite soma, glandularia, evolution