REPRODUCTION OF ASELLUS AQUATICUS. 341 



but there are definite and very effective methods of. doing this. The first 

 one to be noted is the lowering and raising of the oostegites themselves. 

 The plates slowly lower a little way. The appearance is as though the 

 brood-pouch was swelling. Then the plates return rather quickly to the 

 normal position. A considerable change of water is caused by this method,, 

 and the eggs are seen to shake about during the operation. If the animal is 

 undisturbed, this operation may be repeated about 8 times a minute for 

 10 minutes or longer ; and then a period of rest before restarting another 

 set of pulsations. By the use of powdered carmine the effectiveness of the 

 method was easily demonstrated, and it was clear that the flow of water was 

 from behind forwards. The carmine was sucked in at the hinder end of the 

 pouch and, after several movements of the plates, was entirely removed. 



The 1st pair of legs is carried bent inwards at the joint between the 

 basipodite and the ischiopodite, to form two elbows which repeatedly jerked 

 backwards into the brood-pouch whilst the plates were being lowered and 

 raised. By fastening the legs so that they could not do this, I found that 

 they acted as guards at the exit of the brood-pouch. By their action thev 

 prevent eggs escaping with the outgoing rush of water. Quite a number of 

 eggs escaped whilst the legs were not in action, but under normal conditions- 

 I have never seen them escape (Pis. 25 & 26. figs. 2 &, 10). 



There is a further method of changing the water in the brood-pouch. It 

 is a less vigorous method than the one just described, and is not seen during 

 the movements of the oostegites. It is in the periods of rest between the 

 pulsations of the brood-pouch that the flapping movements of the maxilli- 

 pedes are noticed. The maxillipedes are broad and plate-like, and are 

 situated just in front of the 1st pair of oostegites. By their flapping 

 movement they draw a steady stream of water through the brood-pouch. 

 PI. 25. fig. 5 shows one of these maxillipedes taken from a female with a 

 brood- pouch packed with eggs. If it is compared with a maxillipede of a 

 non-egg-carrying female or of a male (PI. 25. fig. 4), it will be seen that 

 a coxal lobe is present. This lobe is covered with feathered setfe and 

 projects a little way into the brood-pouch (PI. 26. fig. 10). The addition of 

 this feathered lobe will make the maxillipede more effective as a current 

 producer; but their position and structure make them effective guard? 

 against the escape of eggs during this operation. The 1st pair of leg? 

 only come into action during the more vigorous method. 



This coxal lobe is only found on the maxillipedes of a female with brood- 

 pouch. In a female taken from the marriage-clasp, and about to cast the 

 cuticle; the new chitinous cuticle, with the coxal lobe, could be seen under 

 the old cuticle of a normal maxillipede, ready to be pushed out when the 

 old cuticle was cast. 



The eggs take about a month in their development. The tiny Aselli 



LINN. JOURN. ZOOLOGY, VOL. SXXIV. 2G 



