INTERNAL ANATOMY. 109 



the lowest, except tlie Tyroglyphidse, '24 in T hi/as 

 petropliilm, one of the Hydrachnidse. 



Nalepa says that the brain in T. siro is about 70 // 

 long, and its greatest breadth is about 50 /u. The 

 sub-cesophageal ganglion he says is an elongated, 

 star-shaped plate ; whose ventral surface is slightly 

 arched. He says that the ganglion-cells are apparently 

 about 1 J fi in diameter, but are very difficult to measure. 



The nerves, except the four great pairs running to 

 the legs, are, as before stated, extremely difficult to 

 trace ; these four pairs are very easily seen ; they do 

 not arise merely from the surface of the sub-ceso- 

 phageal ganglion, but may be traced far into its 

 substance (PL A, fig. 8, n 1, n 2, n 3, n 4) ; they do not 

 appear, so far as I can see, to be accompanied by the 

 smaller accessory nerves, one of which is usually found 

 with each principal leg-nerve in Trombidium, Bdella, 

 Thy as, Hydrodroiita, etc. 



Behind the pair of nerves serving the fourth pair of 

 legs is another pair of large nerves, the starting of 

 which it is quite easy to see (PI. A, fig. 8, ncf), but I 

 could not trace them quite to my satisfaction in the 

 Tyroglyphida3, although they undoubtedly ran into the 

 abdomen ; Nalepa agrees in this, but he does not enter 

 into further detail of their course. In Bdella Hasten 

 and Thyas petrophilus, however, where the nerves are 

 much larger and more easily seen, I was able to dissect 

 out the brain with most of the principal nerves and 

 many of the lesser ones attached ; and to follow these 

 nerves by dissection, as well as by sections, for a con- 

 siderable distance ; in most cases to the organs they 

 innervated ; where we find in the Tyroglyphidas a 

 nerve apparently homologous to a nerve which was 

 traced in Bdella and Thy as I think it is not going 

 too far to suggest that they probably serve the same 

 purpose, although the Acari belong to different families. 

 Now both in Thyas and Bdella there is a pair of nerves 

 apparently homologous with this pair of abdominal 

 nerves in the Tyroglyphidge ; it is the posterior pair of 



