THE REPRODUCTION OF ASELLUS AQUATICUS. 335 



Notes upon the Reproduction of Asellus aquaticus. By Ernest E. Unwin, 

 M.Sc. (Leeds). (Communicated by the Rev. T. R. R. Steering, M.A., 

 F.R.S., F.L.S.) 



(Plates 25, 26.) 



[Read 5th June, 1919.] 



Introduction. 



Some years ago when beginning an enquiry into the life and structure of 

 the terrestrial Isopoda, I started a general examination of the order ; and 

 the fact that Asellus aquaticus was common in all ponds and streams and 

 easily kept in captivity, led me to spend not a little time upon its life and 

 structure. I have kept it more or less under observation since. 



Many ii.vestigators have paid attention to this fresh-water isopod. Leydig, 

 Schneider, Rosenstadt, Kimus, and more recently Wege and Kaulbersz, 

 have published valuable papers *. Kimus deals exclusively with the struc- 

 ture of the branchial pleopods in a comparative study of the branchiae of 

 aquatic isopods. Wege uses Asellus for experiments upon regeneration of 

 appendages. Kaulbersz, in a long and interesting paper, describes a large 

 number of experiments dealing with the behaviour of Asellus under varying 

 conditions of light and chemical stimuli, as well as some notes upon 

 regeneration, reproduction, and moulting. 



The notes which follow are a contribution to a fuller knowledge of 

 reproduction in the Isopoda. 



Reproduction. 



There is an intimate relation between the processes of reproduction and 

 the change of cuticle. Both copulation and egg-laying are heralded by the 

 ecdysis of the female cuticle. It will be best, therefore, to describe this in 

 brief. The cuticle is always cast in two pieces: first of all the hind-half 

 and then, after a short interval, the fore-half. This interval has usually 

 been about 24 hours, but I have records varying from 8 hours to 3 days. 



Both Kaulbersz and Wege confirm the normal interval; the latter writer 

 has compiled an elaborate table in which a day is the usual interval, and he 

 mentions that the normal time is from 12 to 24 hours. 



The splitting is always between the 4th and 5th thoracic segments. 1 

 have examined a large number of cast [cuticles and have not, as yet, found 

 an exception to this. 



* See "Bibliography," p. 342. 



