1895.] HrDBACHNlD FOtWD IN CORNWALL. 185 



The dorso-ventral muscles and the excretory organs pass 

 through the hollow of this square. It is not very difficult to 

 imagine how this state of things, exceptional as it is, arose ; if we 

 turn to a different family of Acarina, the Gamasidse, we shall find 

 usually a very small ventriculus, with a pair of small anterior and 

 one or two pairs of long posterior cfecal diverticula : in the 

 Orihatidse we have a larger ventriculus and a single pair of 

 posterior diverticula, often very long. If we suppose a creature 

 with the small ventriculus of the Garaasidae and the single pair 

 of long cseca found in the Oribatidse, and suppose these caeca 

 pressed together at their posterior ends, as in Henkin's de- 

 scription of Tromhidium, we have only to suppose that these 

 two cseca coalesce at their point of contact, and that, the walls 

 becoming obliterated, a continuous lumen is formed, and we have 

 the venti'iculus of Thyas petrophilus. It is true that we must 

 imagine the coalescence to be so perfect that not a trace of the 

 origin from two paired cseca is left. 



The hind gut and excretory organs must be treated as one 

 question ; authoi's are not by any means agreed upon the con- 

 struction or homologies of these parts in the Hydrachnidse and 

 Trombidiidse. Croneberg describes the ventriculus as a viscus 

 closed posteriorly, and not having any connection with anything 

 in the nature of an anus : he says, in fact, that in all the species 

 of both families which he investigated there is an entrance for 

 food into the ventriculus, but not any organ for the discharge of 

 fsecal matter ; it must be confessed that at first sight this appears 

 improbable. Croneberg draws an opening on the ventral surface 

 of Eijlais between the fourth pair of legs which has the appearance 

 of an anus, and which in his figures he letters " an." although 1 

 am told that anus would not be quite a correct translation of the 

 Russian expression in his explanation of his plates. In his German 

 paper on Tromhidium, however, he calls the opening anus 

 "After"; but in both papers he says that it has not any con- 

 nection with the alimentary canal but only with the excretory 

 organs, and he draws and describes a single clavate sac over- 

 lying the ventriculus in the median line, but ending blindly in 

 front, and not having any entrance into the ventriculus, but 

 passing along the median line of its dorsal surface and bending 

 down behind it to the opening " an." on the ventral surface ; this 

 organ he describes as being filled with the white matter so abun- 

 dantly found in the Malpighian vessels of many other Acariua, 

 e. g. the Gamasidae. Croneberg's view by no means agrees with 

 the previously expressed opinions of Pagenstecher, who considered 

 Croneberg's excretory organ to be the rectum, and characterized 

 Dujardin's earlier suggestion that an Acarns might be without an 

 anus as an excursion into the realms of fancy. Henkin found in 

 Tromhidium an arrangement similar to that described by Crone- 

 berg, but although he could not find any connection between the 

 ventriculus and Croneberg's excretory organ he thought that there 

 jnust be one a,nd that the latter must be regarded as the hind gut. 



