Variations in Uropoda. By A, D. Michael. 147 
a door which closes ifc, yet this projection never enters, but always 
remains wholly external upon the ventral surface. A projection 
of the same nature exists in the same situation in almost all 
species of Uropoda, but the form differs greatly in the respective sorts ; 
in some species the projection is so thin and transparent that it 
usually is not noticed when it lies against the body of the creature. 
As to the use of this projection, it might be suggested that it is to 
prevent the genital plate from passing too far into tho genital open- 
ing ; but this can hardly be so, because within that opening is a 
ledge (fig. 13, df) running all round the opening except its 
posterior, or hinge, side, and which manifestly fulfils this oflSce. The 
function of the projection, therefore, probably is to form a handle 
which may enable the genital plate to be pulled downwards from 
the exterior, turning on the ginglymus hinge at its posterior edge, 
and thus exposing the genital aperture. 
Inside the upturned edge of the genital plate lies a mem- 
branous spoon-shaped piece (fig. 2, g m), which roughly follows the 
shape of the plate, but is not a lining in the sense of being 
attached by its whole surface. 
When the genital plate has been removed, or turned down out of 
the way, it may be seen that the genital opening is the entrance to a 
chamber of which the genital plate, when closed, forms the floor ; this 
chamber is the vestibule ; a large portion of its roof, lying along the 
median line, is composed of the organ which I propose to call the 
perigynum (figs. 2, 3, 4, 5, &c., pg), and which, I think, has not 
previously been described. From the edge of the perigynum a flexible 
membrane extends outward, forming a dome, the lower edge of the 
membrane being attached round the genital opening. It must not 
be supposed, however, that the domed shape is the ordinary condition 
of the chamber ; quite the contrary is the (;ase. The perigynum is a 
stiff structure with chitinous strengthenings, the membrane is extremely 
flexible ; thus the perigynum, when not drawn upward by muscles, 
falls until its lower surface approaches the genital plate, the mem- 
branes folding to allow it to do so ,* thus in its usual position the 
membrane runs upward from the edge of the perigynum, not down- 
ward. If a hair be introduced through the genital opening and 
pushed gently against the perigynum, then that organ will be raised, 
extending the membrane, and disclosing the true shape and nature 
of the vestibule. It will be seen hereafter that the perigynum is 
provided with muscles specially adapted to do what has been arti- 
ficially done by the hair. 
In addition to the great genital aperture there are two smaller 
openings into the vestibule, both from within ; one is the entrance to 
the receptaculum seminis, the other the entrance to, or perhaps it 
should rather be said the exit from, the vagina ; the latter is usually 
closed by, and always hidden by, the perigynum, until disclosed by 
dissection. 
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