184 MR. A. D. MICHAEL oif AN [Mar. 5, 



" 6, b " on the plate, and I think it is a misprint for " n, n " ; I am 

 not able to read Russian, 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 Trombidiv.m in 1879, Croneberg draws the 

 azygous nerve starting from the brain and passing above the 

 cBsophagus, although he does not say where it goes to. 



The oesophagus (fig. 20, 23, ce.) is a tube about '25 mm. long 

 and of about even dimensions throughout ; it rmis right through 

 the brain in the usual manner, and enters the lower part of the 

 ventriculus in the median line about "02 mm. 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 Trombidiidae, 

 consist of a broad viscus, flattened dorso-ventrally, occupying 

 the greater part of the dorsal surface of the creature, and 

 fiu-nished either with numerous shortish, caecal, mostly paired 

 diverticula which arise from the dorsal surface and edges of the 

 ventriculus, as shown by Croneberg for EjjJctis, or with a smaller 

 number of diverticula of somewhat larger dimensions, as found 

 by Schaub and Henkin in Hydrodroma and Tromhidmni. 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 

 Tliyas petropliilvs 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 •! mm. and a 

 perpendicular dorso-ventral measurement of about -11 mm. in 

 its deepest place, ^. e. where the oesophagus enters. There is an 

 unpaired median caecum {ca.) about "15 mm. in length, which 

 projects forward and slightly upw"ard ; 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 hue. 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 

 •] mm. from the posterior end of the creature, maintaining their 

 full width throughout; they have two shallow in-egular lobes on 

 then* outer edge and a tendency to a lobate projection of the 

 posterior corner; they then turn inward andi 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 

 througJiout, 



