180 ME. A. D. MICHAEL ON AN [Mar. 5, 



is also occasionally an irregular central protoplasmic mass, and the 

 whole is joined by delicate threads. It seems not improbable that 

 the absence of chitinization from the exterior tunic of these glands 

 may be correlated with the much greater chitinization of the 

 external cuticle in Thyas than in Hydroclroma. Sehaub and Haller 

 appear to have found that the mouth of each of these glands was 

 surrounded by a thick ring of chitin, and was in connection with 

 a more or less triangular chitinous sclerite bearing a small spine, 

 which may be regarded as protecting the opening ; neither of these 

 conditions, however, is to be found exactly in Thyas petrophilus. 

 The dermal glands of this species discharge to the exterior either 

 through a largish central hole in one of the numerous smaller 

 chitinous plates in the cuticle (fig. 25, os.), each of which plates 

 bears a small hollow spine (p«.), or else at the edge of a plate, usually 

 in the former manner ; the sclerite is, however, distinctly a plate 

 with numerous pores, which bears both the hair and the mouth of 

 the gland; there is not one solid ring-like ridge surrounding the 

 mouth and another triangular ridge supporting the hair. I have 

 not been able to make certain of any really definite connection 

 between the dermal glands and these smaller dermal plates, as it 

 seems to me that the number of plates does not agree with the 

 number of glands, and that some of the plates have not the 

 central opening ; it is, however, extremely difficult to be absolutely 

 sure on this point. 



In a few instances I have found near where the duct emerges 

 a minute and extremely delicate membranous sac within the gland, 

 which sac contains an almost globular structure formed of open 

 irregular network, which stains deeply (fig. 26). 



The Alimentary Canal and Excretory Organ (Plate VIII. figs. 14, 15 ; 

 Plate IX. figs. 23, 27). 



I join these two systems, because, in efEect, it is impossible 

 properly to separate them in the Hydrachuidse, and indeed in 

 some other families of the Acarina, e. y. the Gamasidae. 



The alimentary canal in Thyas lietrophilus differs considerably 

 from everything which, to my knowledge, has been described in the 

 family, or indeed in the Acarina at all ; although undoubtedly it is 

 a modification of the same general plan. 



At the entrance from the mouth to the pharynx I find organs m 



which I suppose are those described by Sehaub as " palpenartige I 



Gebilde " ; I am not, however, able to regard them as of the nature m 



of palpi ; they seem to me, in my species at all events, to be small m 



masses destined either to fit together very closely, and indeed to 

 interlock, and thus form a valve closing the entrance to the 

 pharynx, or else to be separated at the will of the creature, thus 

 completing the pharynx in its office as a sucking apparatus. 



The pharynx itself with its muscles (figs. 24, 27,p7i.) has the 

 same lanceolate form shown by Sehaub in Hydrodroma, and 

 has an average length of about "15 mm., by a breadth, in its 



