Mr. A. Tulk on the Anatomy of Phalangiuni Opilio. 251 



dilated, and exhibits two small, oval and concave plates {ps), 

 situated upon its upper surface, and diverging obliquely from the 

 dorsal groove upon either side. They are darker in colour than 

 the rest of the body, and separated internally from each other by 

 a narrow space. Their outer margin, dark brown, is prolonged 

 in the middle in a triangular shape, and curved towards the me- 

 dian line. The second piece of the male organ or glans {g) is 

 articulated to the former in a ginglymoid mannci", and rests ob- 

 liquely downwards upon the two above-mentioned plates. It is 

 widest behind where it projects in a rounded base, which has two 

 slender elevator muscles attached to it, is concave upon its upper 

 and lower surface, and fm-nished at the apex with a small, acute, 

 shghtly bent and moveable hook, at the base of which, interiorly, 

 is a minute triangular aperture for the exit of the seminal secre- 

 tion. 



The whole of this penis is contained in a sheath (PI. IV. fig. 23. 

 sh) of similar contexture, which is situated partly in the hollow 

 of the sternum [s), and partly against the ventral parietes, being 

 about twice the length of the former. It is broader than the or- 

 gan it incloses, and being more or less transparent, admits of the 

 outline of the penis [j)) being very distinctly seen through its 

 parietes. Though described by Treviranus as a simple sheath, 

 this organ consists of two elongated pieces with well-defined 

 margins, connected to each other by a membrane, which is very 

 easily torn through in dissection if care be not used, and gives 

 the idea at fii-st, of the structure in question being formed by a 

 couple of separate valves. The lower of these pieces is carinate 

 in form, to adapt itself to the corresponding surface of the penis 

 against which it rests, and is continuous behind with the inferior 

 part of the margin of the opening in the base of that organ ; its 

 lateral borders become thicker towards their anterior extremity, 

 and terminate by cmwing outwards as two horny projections or 

 hooks [h], which are connected upon either side to the edges of 

 the sternal plate, with an inflection of which the inferior part of 

 the sheath is confluent anteriorly [a) . The second portion of the 

 sheath, attached to the remaining edge of the urethral opening, 

 extends almost flat over the upper grooved surface of the penis, 

 and ends in front by a rounded free border (*). From the open- 

 ing between the two conjoined pieces the glans of the male or- 

 gan protrudes. 



The relative position of the above parts " in situ" is precisely 

 as follows : — Inferiorly, lying wdthin its sheath in the concavity 

 of the sternum, is the intromittent organ. Above, covering the 

 anterior half of the sheath, the divergent angles of the latter, and 

 the glans penis being partially visible, is the cluster of seminal 

 Immediately behind the latter is the horny portion of 



S2 



