TNTKKNAL ANATOMY. 107 



commissures, standing perpendicularly one on each 

 side of the oesophagus, and joining the two ganglia. 

 In most of the Acarina, however, the commissures 

 have lost all distinctive character ; it is impossible to 

 say where they commence or finish ; the supra-oeso- 

 phageal ganglion and the sub-oesophageal ganglion 

 appearing to be joined to one another and forming a 

 continuous substance. In some families of Acarina, of 

 which the Hydrachnidse are the best example, the 

 extreme fusing of cephalothorax and abdomen into 

 one undivided mass, which is stated in so many text- 

 books to be characteristic of the Acarina generally, 

 but which is really only found in a few families,* is 

 accompanied by an entire fusing of the supra- and 

 sub-oesophageal ganglia ; so that the whole brain, 

 ganglia, and commissures form a single, almost 

 globular mass ; which is pierced centrally, but rather 

 obliquely, by the oesophagus. t In some other Acarina, 

 as, for instance, B dell a Basteri, the supra- and sub- 

 oesophageal ganglia quite retain their character of 

 separate ganglia, although they are joined together, 

 and no line of demarcation can be seen in transverse 

 sections through the two .{ In such Tyroglyphidae as 

 have been investigated the brain may be fairly said to 

 be in a condition intermediate between these two 

 formations. In Glycyphagtts platygaster (PI. A, fig. 10), 

 seen in sagittal median section, the whole brain is 

 almost wedge-shaped, except that the sub-oesophageal 

 ganglion bears a distinct rounded swelling on its dorsal 

 surface immediately adjoining the aperture for the 

 passage of the oesophagus ; the supra- and sub-oeso- 

 phageal ganglia being about equal in length. In 

 Hericia liobini (PL C, fig. 1, gso, gsu) the shape re- 



* See p. 49 of this book. 



t R. von Schaub, "Ueber die Anatomie von Hydrcdroma," in ' S. B. 

 Ak. Wien,' 1888, Bd. xcvii, Abth. i, p. 98; Michael, "A Study of the 

 Internal Anatomy of Thy as petrophilus" in ' P. Zool. Soc. London,' 

 1895, p. 199. 



J Michael, " The Internal Anatomy of Bdella" in ' Tr. Linn. Soc. 

 London,' 1896, vol. vi, pt. 7, p. 497, pi. xli, fig. 13. 



