1895.] HTDEACHlJfID FOTTNT) IN CORNWALL. 201 



between Schaub's description and Thyas petrophihis is that he shows 

 two fine branches as springing from the optic nerre some time 

 before the final division into two ; 1 find three such branches quite 

 plainly visible in dissections. Schaub says that these branches go 

 to sense-organs in the dorsal shield ; I have not been able to find 

 such sense-organs in my species, which has not the peculiar dorsal 

 plate of Hydrodroma in which they are situated, and I have not 

 been able to trace where the three fine branches in my species go 

 to : I have two or three dissections sho^^'ing the whole course of the 

 optic nerve from the brain to the eye, and showing these branches 

 for some distance; but I have not been able to trace them to 

 their destinations and I cannot follow them in the sections. 



The fourth pair of nerves from the upper ganglion is a pair of 

 verj' thin nerves from near the posterior edge of the brain (fig. 20, 

 nu.), and which lie above and between the nerves serving the 

 fourth pair of legs and the genital nerves : these nerves are 

 extremely fine and difiicult to trace, but are certainly present ; I 

 have not succeeded in ascertaining what oi'gans they innervate. 



The pair of nerves which proceed from the level of the oeso- 

 phagus, so that it is hard to say whether they are supra- or sub- 

 oesophageal, are shown at fig. 20, np. ; they are long and substantial 

 nerves and I have sections showing them well in their entire 

 length. Each nerve, shortly after leaving the brain, forms a 

 ganglionic swelling and then diminishes to its former size : about 

 halfway between the brain and its destination it sends a very 

 small branch downward ; I have not been able to trace this to its 

 destination. Some distance from its termination the principal 

 nerve divides dichotomously, sending one branch {np. 1) forward 

 and upward to the palpus, and the other {np. 2) forward and 

 downward to the maxillary lip ; a short distance before reaching 

 which it forms a small ganglionic swelling. 



This nerve is probably homologous with Schaub's nerve " ant," 

 which he says serves the palpi and mandibles ; of course this may 

 be so in his species, but as the palpi are maxillary palpi and the 

 maxillary lip in Acarina is formed of the fused maxillse, the distri- 

 bution to palpi and maxillary Hp seems more what might be 

 expected than that to palpi and mandibles. Schaub considered 

 that the palpi were innervated from the su])ra-cesophageal ; Crone- 

 berg from the sub-oesophageal ganglion. Xalepa (in Tyroglyplius) 

 considered that the maxillae were served by the sub-, and the max- 

 illary palpi by the supra-oesophageal ganglion. In the present 

 species it is, as before stated, impossible to say which ganglion the 

 nerve belongs to. 



Of the nerves clearly proceeding from the sub-oesophageal part 

 of the brain-mass there are, firstly, the four pairs of great nerves 

 proceeding to the four pairs of legs (fig. 20, n 1, n 2, n 3, n 4) ; as 

 to the existence and position of which all WTiters are agreed ; but 

 all have hitherto described and figured them as unbranched 

 nerves, at least no one has described any branches, although their 



