ADAMS: OBSERVATIONS ON THE PAIRING OF ARlON ATER. 117 



forth to enjoy it, swarming over the roads and hedges. At 3-30 p.m., 

 I was returning from Henfield to Seeding, examining the snails along 

 the low stone and turf wall, which bounded one side of the tarred high 

 road, when I came upon a large colony of A. ater, extending for 150 

 yards along the wall and also on the road. I saw at once that many 

 were intent on pairing, so I proceeded to make notes and sketches, 

 and as I stayed till darkness rendered observation impossible and 

 examined more than twenty couples, I was enabled to follow the per- 

 formance in all stages. At dark I left the spot, but returned in 

 half an hour to find that the slugs had nearly all disappeared. 



The sequence of events was the same in all cases, varying only in 

 the duration of time of the different stages. The procedure was as 

 follows : — One slug would overtake another; the pursuer would begin 

 to eat the slime from the caudal extremity of the other, which would 

 then turn and reciprocate the act. Then would ensue a circular 

 procession similar to that in the case of Z. maxinnts, but at a much 

 slower pace, and without any flapping of the mantle usual with that 

 species. The processions that I timed lasted from ten minutes to 

 an hour and a half When the procession was completed, each would 

 draw up alongside of the other, so that their genital orifices, from 

 which the lower atria were slightly protruding, were opposed (Fig i). 



Fig. I (nat. size). — A couple drawing alongside each other at the completion of the cir- 

 cular procession, o, o, genital orifices. 



The upper atria were then gradually evaginated through the mouth 

 of the lower atria, which organs seem to act as constrictors or 

 sphincters. The evaginated upper atria meet and adhere, ^'sucker' 



I The inflated organs of a paired couple are joined by a neck of dead cream-coloured mus- 

 cular tissue. The bulbs themselves are semitransparent bluish white. This neck of muscular 

 tissue consists of the ' sucker,' the slightly evaginated spermatheca duct, and the epiphallus. 

 These may be seen clearly if the organs of a paired couple be separated ; a separated organ 

 presents the appearance of fig. 3. 



If the upper atrium as it lies in its natural position be slit open and turned inside out, a 

 projected process, which resembles a schoolboy's 'sucker,' with a corrugated surface, will appear, 

 also two holes, which may be squeezed so as to evert the two ducts communicating with 

 spermatheca and epiphallus, fig. 4. 



In the act of coupling, the 'suckers' fit together as fulcra and holdfasts, while the spermato- 

 phore of the one individual is projected into the spermatheca of the other. 



I have called this process the 'sucker,' as that is apparently its function in keeping the everted 

 organs compactly together during tl)e act of coupling. 



