328 Mr. A. Tulk on the Anatomy of Phalangium Opilio. 



of the trunk, just before it opens externally in the stigma, and 

 passes directly backwards for some distance undivided, and then 

 ramifies into several branches. The two next branches arise quite 

 close to each other at a short interval from the former. The most 

 external runs also backwards for some extent as a single tube and 

 then bifurcates, each branch resolving itself again into twos and 

 threes, so as to form a leash of long tracheae, nearly of equal size 

 and parallel to each other. The internal branch divides, not so 

 far as the others from its point of origin, into smaller tubes, and, 

 directed more towards the median line, supplies, with them, the 

 abdominal viscera and generative organs. 



The fourth branch (/), issuing from the main trunk at an acute 

 angle, is remarkable for its size, which is nearly half the width of 

 that of the latter, and its course, like it, is forwards and in- 

 wards, but converging more towards its fellow of the opposite 

 side. After quitting the main trunk it gets beneath its level, and 

 runs to the posterior margin of the thoracic ganglion, beneath 

 which it passes, and there terminates abruptly by a rounded cla- 

 vate extremity, from which arise three branches. The most pos- 

 terior of these takes a peculiar direction ; it passes transversely 

 inwards, crossing the medio-abdominal nerve and anterior part 

 of the sheath of the ovipositor in the female as at fig. 33, and 

 anastomosing with a similar tube from the opposite side, forms a 

 very delicate arch just behind the thoracic ganglion. From the 

 anterior side of this arch (which has been spoken of above as the 

 transverse tracheal vessel) two long curving and slender tubes 

 proceed forwards, and are probably continued into the maxillary 

 palpi. Treviranus has figured and described an azygos tube as 

 arising between these two and passing forwards, while another 

 given ofi" opposite takes a backward course. I have been unable 

 to detect either of these branches, but have met in some specimens 

 of P. Opilio with two tracheae arising, nearly opposite to the an- 

 terior pair, from the arch and entering the abdominal cavity. The 

 two other branches given off from this large trunk run beneath 

 the thoracic ganglion, their terminal ramifications emerging upon 

 either side the optic, and supplying adjacent organs. The main 

 trui^k of the tracheal system furnishes but one other branch from 

 its inner aspect which ramifies near to the optic ganglion, and 

 then gradually diminishes in size to form a terminal tube {tc), 

 which is intended to convey air into the interior of the chelicerse, 

 and from the outer side of which several smaller, more or less 

 parallel vessels arise. The branches from the outer side of 

 the main trunk arise in pairs, of which there are four [ct), one 

 of each passing along the upper surface of muscles in the coxal 

 joints to supply the legs, while the other goes to the parietes and 

 viscera of the cephalo-thorax. From the divergent position of the 



