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Show 1895.] HYDBACHNID FOUND IN COENWALL. 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 Gamasidae, w e shall find usually a very small ventriculus, with a pair of small anterior and one or two pairs of long posterior caecal diverticula: in the Oribatidse we have a larger ventriculus and a single pair of posterior diverticula, often very long. If w e suppose a creature with the small ventriculus of the Gamasidae and the single pair of long caeca found in the Oribatidae, and suppose these caeca pressed together at their posterior ends, as in Henkin's description of Trombidium, we have only to suppose that these two caeca coalesce at their point of contact, and that, the walls becoming obliterated, a continuous lumen is formed, and w e have the ventriculus of Thyas petrophilus. It is true that w e must imagine the coalescence to be so perfect that not a trace of the origin from two paired caeca is left. The hind gut and excretory organs must be treated as one question; authors are not by any means agreed upon the construction or homologies of these parts in the Hydrachnidae 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 faecal matter ; it must be confessed that at first sight this appears improbable. Croneberg draws an opening on the ventral surface of Eylais 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 Mould not be quite a correct translation of the Russian expression in his explanation of his plates. In his German paper on Trombidium, however, he calls the opening anus "After"; but in both papers he says that it has not any connection with the alimentary canal but only with the excretory organs, and he draws and describes a single clavate sac overlying 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 abundantly found in the Malpighian vessels of many other Acarina, e. g. the Gamasidae. Croneberg's view by no means agrees with the previously expressed opinions of Pagenstecher, w h o considered Croneberg's excretory organ to be tbe rectum, and characterized Dujardin's earlier suggestion that an Acarus might be without an anus as an excursion into the realms of fancy. Henkin found in Trombidium an arrangement similar to that described by Croneberg, but although he could not find any connection between the , ventriculus and Croneberg's excretory organ he thought that there must be one and that the latter must be regarded as the hind gut. |