Mr. A. Tulk on the Anatomy of Phalangium Opilio. 329 



two anterior pairs of legs from the two posterior^ there is an in- 

 terval between the two middle pairs of tubes, greater than be- 

 tween the first and second, or third and fourth pairs. Besides the 

 tracheal vessels above enumerated, their minute subdivisions are 

 distributed most extensively throughout the body, some passing 

 within the body of the penis into the ovipositor, upon either side 

 of the pharynx between its muscles, around the circumference of 

 the ovary, and among the seminal caeca, &c. 



As regards the structure of the trachese in Phalangium, I have 

 been unable to detect the existence of a serous or mucous coat 

 distinct from their spiral fibre, but as the coils of this must have 

 some means of connexion, the mucous coat may possibly serve that 

 purpose, in place of being a simple lining membrane to the tube. 

 There is an appearance in these vessels similar to what is met 

 with in the vesicles of insects, and which is due to the presence 

 of an immense number of round spots, which look at first like 

 small openings, and are scattered most abundantly, without any 

 arrangement, over the main trunks, but more sparingly upon the 

 branches, decreasing as they further subdivide. Examined under 

 a power of 500 linear, they are found to consist of two kinds : the 

 larger have a well-defined margin, are least connected to the spi- 

 ral fibre, projecting in many instances from its surface, and con- 

 tain in their interior nuclei, which present a slight hyaline tint. 

 At the points where they occur the spiral fibre is uninterrupted. 

 The smaller spots are very minute and dark, and placed between 

 the coils of fibre, appear like simple nuclei, around which a cell 

 had not yet been developed. This structure, upon the real na- 

 ture and use of which it is very difficult to decide, proves, at all 

 events, that these spots are not partial dilatations of the air-tube, 

 as has been believed by some writers. 



The spiracles are narrow elongated slits, dividing the bottom of 

 the tegumentary fold between the posterior pair of coxae and the 

 abdomen. Their posterior margin is inverted to form a triangular, 

 obtuse, deflexed and horny plate {p), projecting into the abdo- 

 minal cavity at a point corresponding with the origin of the se- 

 cond and third abdominal trachese. It is unprovided with muscles, 

 but is freely moveable to and fro upon the thinner integument 

 that supports it, so that it may be regarded as a kind of passive 

 valve, serving upon the one hand to strengthen the weakest side 

 of the spiracular orifice ; on the other, to yield downwards, so as 

 to admit of the full dilation of the trachea, and again return when 

 the quantity of air admitted into the latter is diminished. It 

 may also serve to prevent compression of the air-tube during the 

 movements of the posterior coxse, and when the body is greatly 

 distended with ova in the female. The outer margin of this plate 



