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Show 184 ME. A. D. MICHAEL ON AN [Mar. 5, " 6, 6 " on the plate, and I think it is a misprint for a n, n "; I am not able to read Eussian, but I fancy they are referred to in the text as " n, n." They appear to be the same nerves as Schaub letters " ant." and says go to the mandibles and palpi. In his subsequent work on Trombidium in 1879, Croneberg draws the azygous nerve starting from the brain and passing above the oesophagus, although he does not say where it goes to. The oesophagus (fig. 20, 23, ce.) is a tube about *25 m m . long and of about even dimensions throughout; it runs right through the brain in the usual manner, and enters the lower part of the ventriculus in the median line about #02 m m . behind the brain. The ventriculus (figs. 14, 23, v.) presents very considerable differences from any hitherto described; those figured by Schaub, Henkin, Croneberg, & c , both for Hydrachnidae and Trombidiidse, consist of a broad viscus, flattened dorso-ventrally, occupying the greater part of the dorsal surface of the creature, and furnished either with numerous shortish, caecal, mostly paired diverticula wiiich arise from the dorsal surface and edges of tbe ventriculus, as shown by Croneberg for Eylais, or with a smaller number of diverticula of somewhat larger dimensions, as found by Schaub and Henkin in Hydrodroma and Trombidium. Of these caeca the posterior median pair turn forward in Schaub's species and backward in the others; Henkin figures and describes them as having their hinder parts pressed together in the median line. In Thyas petrophilus I find near the brain a short anterior tract of the ventriculus, which is rather deeper than it is long; i. e. it has a horizontal antero-posterior measurement of about *1 m m . and a perpendicular dorso-ventral measurement of about *11 m m . in its deepest place, i. e, where the oesophagus enters. There is au unpaired median caecum (cce.) about '15 m m . in length, which project s forward and slightly upward; it has a somewhat clavate distal extremity and lies immediately over and upon the quadrate salivary glands, where they press against each other in the median line. From the sides of this, which I consider the ventriculus proper, two expansions, as wide as the ventriculus proper, run laterally and form a shallow rounded lobe on each side; they then run straight backward and continue up to about •1 m m . from the posterior end of the creature, maintaining their full width throughout; they have two shallow irregular lobes on their outer edge and a tendency to a lobate projection of the posterior corner; they then turn inward and join without showing any sign of demarcation. The result of this is that the whole ventriculus forms what would be called a ring if it were round instead of square. As it exists it is a hollow square with shallow lobes at the angles, rather more strongly-marked lobes on the outer sides, and a single azygous caecal diverticulum in the anterior median line. The lobes vary somewhat in different specimens, but the general plan is the same. The whole hollow-square must be considered to be the ventriculus; the lumen is continuous throughout. |