Mr. A. Tulk on the Anatomy of Phalangium Opilio. 253 



first of these organs (PI. IV. fig. 24. ?/') may be readily seen 

 through the extruded sheath^ where its presence is indicated by 

 a dark hne^ shadowed externally, and may be even drawn into it, 

 when the parts are all in their natural position, by laying hold of 

 the extremity of the penis with a fine pair of forceps. The extru- 

 sion is, I believe, effected by the contraction of the fibres of the 

 corium, which, by diminishing the abdominal cavity, press upon 

 the generative organs from behind and cause them to spring out, 

 while the retraction of the penis, and with it, of course, the sheath, 

 is performed by the two special muscles above described. I may 

 observe, that, owing to the protrusion of the anterior part of the 

 vas deferens, the whole of it is placed in a more direct position 

 in relation to the seminal caeca, and the passage into it, of the 

 secretion from the latter, thus greatly facilitated. 



EXPLx-^NATION OF PLATE IV. 



Fig. 15. Inferior view of the epipharynx, to show the pharynx lodged in its 

 interior, and the commencement of the oesophagus ; A A are the 

 first pair of maxillae with their horny bows ; 2 2, seconji joints of 

 the palpi ; s, the vertical septum. 



Fig. 16. Anterior opening into the pharynx ; dp is the deflexed termination 

 of the dorsal plate of the latter, forming at its apex a corneous lip ; 

 m m are muscular fasciculi. 



Fig. 17. The alimentary apparatus, seen from its upper surface, th« pharynx 

 and oesophagus having been removed. 



Fig. 18. The same, from below. The corresponding caeca are lettered the 

 same in both these figures. 



Fig. 19. An oblique transverse section of the abdominal viscera through their 

 middle in a female Phalangium Opilio. The epidermis has been re- 

 moved, leaving only the membranous layer m'm' beneath it, with 

 the fibres of the corium, m rn, resting thereupon ; the relations of 

 the ovarium, O, and ovipositor, or, are also seen. 



Fig. 20. Oval mass of faeces, inclosed in its membranous* capsule. 



Fig. 21. The male generative organs dissected out, and seen from their 

 under surface ; c c are the pair of latero-inferior long caeca ; a, the 

 anus. 



Fig. 22. The penis, the sheath having been removed, viewed from above. 



Fig, 23. The same, lying within its sheath, sh, in the concavity of the ster- 

 num, s. 



Fig. 24. The sheath and penis extruded from the sexual opening, seen from 

 their inferior surface; 1, 2, 3, 4, coxae of legs ; B is the second 

 pair of jaws ; D, appendages to second coxal joints. 



Fig. 25. A male P. Opilio, viewed from the side, with the sexual organs ex- 

 truded. The legs have been cut off, through the extremities of 

 the third joints. 

 The preceding figures are magnified as in Plate III. 

 [To be continued.] 



* I have been unable to discover with the microscope any traces of a 

 distinct structure in the membrane forming this sac, the texture of which is 

 smooth, homogeneous, and so thin as to be transparent. Its. existence has 

 been very constant in the many specimens of Phalangium which I have ex- 

 amined. 



