The structure of the internal sexual organs is very remarkable in the Thysanura. In _Campodea_ there is one extremely large, simple tube on each side of the body. In _Japyx_ there are seven small tubes on each side, placed one in each of the successive abdominal segments, and opening into a common duct. In _Machilis_ there are also seven tubes opening into a common duct, but the arrangement is no longer a distinctly segmental one. In _Lepisma_ there are five egg-tubes on each side, the arrangement being segmental in the young state but not in the adult condition. In _Campodea_ nutrient cells alternate with the eggs in the tubes, but this is not the case in the other families. Fig. 94 shows the ovaries in three of the Thysanura; in the drawing representing this part in _Machilis_ (C), one of the two ovaries is cut away for the sake of clearness. The male organs in _Campodea_ are very similar in size and arrangement to the ovaries, there being a single large tube or sac and a short vas deferens on each side of the body. In _Japyx_ there is a sac on each side, but it is rendered double by a coecum at its base, and there are long and tortuous vasa deferentia. In _Lepisma_ there are three pairs of coeca on each {188}side, segmentally placed and opening into a common duct. In _Machilis_ there are three retort-shaped sacs on each side opening near one another into a common duct, the vasa deferentia are elongate, and are very curiously formed, being each double for a considerable length, and the separated portions connected at intervals by five transverse commissural ducts. [Illustration: FIG. 94.—Ovaries of Thysanura: A, of _Campodea_; B, of _Japyx_; C, of _Machilis_. (After Grassi and Oudemans.)] One of the characteristic features of Insect structure is the restriction of articulated legs to the thoracic region. In the Thysanura there exist appendages occupying a position on the hind body somewhat similar to that of the legs on the thorax. These appendages are quite small bodies, and are placed at the hind margins of the ventral plates of the abdomen, one near each side; they are connected by a simple joint to the sternite and are provided with muscles. They are found in _Campodea_ on segments 2 to 7; in _Lepisma_ on 8 and 9, in the allied _Nicoletia_ on 2 to 9; in _Japyx_ on 1 to 7, being, however, more rudimentary than they are in _Campodea_. In _Machilis_ they attain perhaps their greatest development and exist on segments 2 to 9; moreover, in this genus such appendages occur also on the coxae of the second and third pairs of thoracic legs. Oudemans thinks they help to support the abdomen, and that they also assist in leaping; Grassi considers that they are supporting agents to some extent, but that they are essentially tactile organs. He calls them false legs "Pseudozampe."