38 THE SCOTTISH NATURALIST 



Each plate really consists of two fused together, so that there 

 are actually eight plates one for each leg. These plates are 

 termed epimera, a word which is used in quite a different 

 sense from that used by Huxley {Crayfish, vol. 28, Inter. 

 Sc. Series). As the muscles which actuate the leg spread 

 out in fan shape from it to be anchored on the farther 

 edge of the plates, it is obvious that these must be of 

 appropriate size and strength. Accordingly, we find that in 

 those water mites which are soft-skinned, the plates are well 

 defined and have strong margins, and that in those that are 

 hard-skinned, the plates may not be so well defined. Con- 

 siderable variety in the epimera may be noted in different 

 species ; in some species, the epimera may even be fused 

 into one plate, covering from about half to the whole of the 

 ventral surface. The capitulum lies in the bay formed 

 between the first pair of epimera. It is small, and has 

 a limited power of movement. Its ventral surface is a hard 

 plate, considered by some writers to represent a pair of 

 maxillae fused together. The walls form a chamber, at 

 the apex of which the mouth is usually situated. The dorsal 

 wall forms a groove within which the mandibles lie. In the 

 halacarids, the groove is wanting. On the dorsal wall also 

 the stigmata or external breathing organs may be noted. 

 On the capitulum there will be observed a pair of palpi, 

 much diversity will be found in the character of these, but all 

 agree in having five segments. Posterior to the epimera is 

 the genital area. Great diversity, incident to species and sex, 

 may be noted ; and again we meet with a term, acetabula, 

 which, more commonly used in another sense, is rigidly 

 confined in hydracarid terminology to the genitalia. As 

 so applied it refers to the glands, represented externally by 

 discs let into the genital plates. These glands may even be 

 everted and become stipitate, but the term is still applied to 

 them. Many varied forms of genitalia are to be met with. 

 In one genus, the vulva has neither plates nor acetabula ; in 

 another, acetabula only are present ; and in others, the 

 acetabula are either embedded in plates or protected by 

 plates hinged to the body at their outer edge, so that 

 they can meet at the centre and cover the acetabula. 



