Mr. A. Tulk on the Anatomy of Phalangium Opilio. 321 



examination, a few of them have become detached from the surface 

 of the membrane. Another peculiarity of this sheath is its very 

 high degree of elasticity, as it may be stretched to three times its 

 natural length without rupture, resuming with a spring its former 

 position and extent when the tension is removed. When within 

 the abdomen, the sheath of the ovipositor is seen to be of a black- 

 ish colour, but this is due to its numerous transverse folds and 

 the spines upon its surface, since the more it is drawn out the 

 lighter it becomes, until the true white colour of both membrane 

 and spines is rendered visible. In the unextruded condition of 

 the ovipositor the spines are situated upon the internal surface of 

 the sheath, lying closely appressed thereon, with their points di- 

 rected forwards ; but when these parts have been everted, which 

 is eifected in a similar manner to those of the male, they stand 

 out externally, giving to the outline of the sheath an irregular 

 surface, very different from that which it had within the abdo- 

 minal cavity. The spines cease at a little distance from the ter- 

 mination of the sheath anteriorly, by a somewhat abrupt trans- 

 verse line, and are continued outwards into two curved and stiflp 

 folds of the membrane upon either side, analogous to the hooks 

 in the sheath of the penis, their use in both sexes being appa- 

 rently to keep the orifice of the sheath open, so as to offer no ob- 

 struction to the egress of its contained organ. The naked mem- 

 brane of the antero-inferior part of the sheath is continuous with 

 an inflection of the sternal lip. 



Besides the sheath already described, there is yet another of a 

 white colour (PI. V. fig. 26. ms) which invests it more loosely, 

 and which is formed by a single layer of ultimate fibrils of large 

 size, arranged in a parallel and longitudinal manner side by side, 

 and united so as to constitute a thin muscular tube over the re- 

 flected portion or true sheath of the ovipositor. Near to the an- 

 terior termination of the latter organ these fibrils diverge from 

 their straight course upon either side, and are aggregated to- 

 gether to form a pair of muscles directed obliquely backwards. 

 The most posterior, the broader and shorter of the two, is of a 

 flattened and triangular shape, and attached by its apex to the 

 sides of the base of the sternal plate ; the anterior is long and 

 narrow, its fibres divergent internally and continuous like those 

 of the preceding with the sheath, its external extremity being 

 blended with that of the posterior muscle and attached along with 

 it. This muscular sheath does not terminate at the posterior end 

 of the ovipositor by a free margin, but is reflected inwards upon 

 itself, part of its fibres being obviously continuous with those of 

 the retractor muscles, while some are probably prolonged onwards 

 over the oviduct to constitute an external contractile coat. After 



Ann. ^ Mag. N. Hist. Vol.xii. Z 



