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



the vas deferens in its surrounding body, lying lengthways upon 

 the upper surface of the posterior half of the sheath, but not ex- 

 tending quite to its base, so as to leave the opening into the penis 

 perfectly visible. The two structures called by Treviranus the 

 ligaments of the sheath, are, without doubt, a pair of retractor 

 muscles (PI. IV. fig. 21. rm). They arise, broadest, upon either 

 side of the opening in the base of the penis, and passing back- 

 wards along the under surface of the abdomen, where they come 

 into relation with the two branches and their ganglia of the medio- 

 abdominal nerve [ng)j diverge to be inserted into the lateral an- 

 gles of the penultimate dorsal arc. Part of the fibres of these 

 muscles are continued onwards from their origin to form a mus- 

 cular sheath, apparently composed of large and detached ultimate 

 fibrils arranged side by side in a single layer, over the sheath of 

 the penis, the vas deferens and seminal tubes. 



Behind the testes, extending across the under surface of the 

 stomach, is a broad intestiniform tube (*), curved in the shape of 

 a Z, and constricted in five or more distinct places, which is sup- 

 posed by Treviranus to belong to the generative system of the 

 male, as it is met with only in that sex, and to perform some 

 function connected with the elaboration (Absonderung) of semen. 

 It is membranous, and contains a granular-looking substance, and 

 terminates by its extremities in two long filiform tubes, which are 

 said also by Treviranus to lose themselves among the blind ap- 

 pendages of the alimentary canal. I have examined the direction 

 of these minute ducts with great care, and find that they pass 

 forwards and curve round the tracheal trunks near to their origin 

 from above downwards, and are lost at the inner extremity of the 

 spiracular groove, where they may probably open externally. The 

 function assigned to this part is thus rendered extremely proble- 

 matical. 



It is doubtless preparatory to the intercourse of the sexes, that, 

 during the autumn, many male specimens of Phalangium may be 

 taken, having the penis and its sheath completely extruded from 

 the cavity in which they before lay concealed (PI. IV. fig. 25.). 

 In this operation the sheath is turned inside out, and the curved 

 and flexible hooks upon either anterior angle turn also over, from 

 above, downwards and backwards, towards the latter end of the 

 process. But not only do the external organs of generation thus 

 undergo a remarkable change of position, but the internal are, to 

 a certain extent, displaced. The horny portion of the vas defe- 

 rens and its surrounding body, appended close to the root of the 

 penis (PI. IV. fig. 23. v^v), and the muscles upon either side of the 

 latter, are thrust out of the abdomen along with that organ, and 

 as it quits its investing sheath enter the latter, but are in contact 

 then with its external surface, changed by eversion, to within. The 



