REPRODUCTION OF ASELLT7S AQUATICUS. 337 



The fourth pair of walking-legs are accessory organs. They will be 

 described later. 



Female. The ovaries are paired and lie dorsal to the alimentary canal and 

 hepatic cpeca. In an immature specimen they are small and show little 

 sign of eggs ; but in a female about to lay eggs, the ovaries are very large 

 and conspicuous, extending the length of the thorax, and have the form of 

 thin-walled bags packed with spherical eggs. The external openings of the 

 oviducts can only be seen at a particular period. When the female has 

 passed through the ecdysis which releases the oostegites, the openings can 

 be seen on the ventral surface at the base of the fifth pair of legs. They 

 are narrow slit-like openings with thickened lips. By means of serial 

 sections through specimens about to lay eggs after being released from the 

 marriage-clasp, I found that these apertures lead into a fairly large spherical 

 cavity in which a mass of spermatozoa could lie seen. It is clear that these 

 cavities function as receptacula seminis. They appear to be lined with 

 chitin and communicate with the ovary of the corresponding side (PI. 26. 

 fig. 12). 



Marriage-clasp and fertilization. — It is a very common thing to find 

 specimens of Asellus in the spring and early summer in pairs, a larger one 

 above carrying a smaller one. These couples are always male and female — 

 male above, female beneath. This is the marriage-clasp, and the larger 

 male always holds the female by a special pair of legs, the 4th pair, which 

 are smaller and slightly modified for this purpose. The 7 pairs of legs 

 normally increase in size from before backward ; but in a mature male this 

 middle pair is an exception to this (PI. 25. fig. 3). 



The three distal segments are modified to form a hook. The carpopodite, 

 with a flattened surface fringed by special spines, is the dorsal arm of the 

 hook, and grips the dorsal surface of the female's thorax. The propodite 

 and the dactylopodite act as the ventral arm of the hook, which bends round 

 the edge of the tergum and grips the ventral surface. The gripping surface 

 of these two segments is similarly flattened and roughened with spines. 

 Each of the legs is able to bend inwards at the joint between the basipodite 

 and the ischiopodite (PI. 25. fig. 3) ; this gives the legs a Z-like arrangement, 

 and the female can be slung in the spiaee between the ventral surface of 

 the thorax and the legs o; the male. By varying the angle between the 

 basipodite and the ischiopodite, the female can be raised or lowered for 

 locomotion or feeding. Kaulbersz does not mention these legs. The 

 position of hooking is not quite constant, but is usually near the middle of 

 the thorax. 



The female does not seem to mind the inconveniences of her position, 

 but resigns herself to the inevitable. "When the male walks forward slowly, 



