36 BeiJurt of the BtUiarzia Miisdo)i ui Egypt, 1915 



the main canal is entirely controlled by the Government, and is sup- 

 plied on a definite rota of six days' flow, succeeded by fifteen days' 

 stoppage, in every three weeks. This rotation began early in April. 

 During the fifteen days' stoppage the water entirely disappeared 

 from the Marg Canal (fig. 11), save where inequalities left small 

 puddles (fig. 12), and we were able to collect stranded shells through- 

 out its length from the drying surface and to recover by sieving 

 those forms that had succeeded in burrowing for some distance into 

 the mud. The species found were : Melania tuherculata, Vivipara 

 unicolor, Cleopatra buUnioides, C. cyclostomoides, Bullinus dyboivski, 

 B. alexandrinus, B. contortus, B. innesi, Pyrgophysa forskali, 

 Lanistes holteni, Planorhis boissyi, P. mareoticus, LimncEa Caillaudi, 

 Bythinia (Gabbia) sennaarica, Valvata nilotica and some bivalves. 



The commonest species were Planorbis boissyi, Bullinus and 

 Cleopatra, spp., the two first-mentioned being the most obvious 

 forms on account of their habit of seeking the surface for air. 



Technique. 

 The various molluscs, wherever and whenever collected, were 

 separated into their different genera and, as far as possible, species, 

 and examined for developmental stages : (a) by direct inspection, 

 (6) by dissection. When an infected mollusc is kept in a glass 

 vessel in clean water for a few days the cercarial forms are 

 frequently discharged naturally by the mollusc, and may be easily 

 seen swimming about in the water if the glass is held against 

 the light and examined with a hand lens. By this method large 

 numbers of the same species can be placed together in a vessel 

 and tested without further technique. If, however, the larval 

 development has not yet reached the stage for the discharge of 

 the cercaria the results are apt to be misleading. Should a 

 positive result be noted, the cercaria can be determined by micro- 

 scopical examination and the infected mollusc isolated and kept 

 alive for further observation. By dissection the earlier stages 

 (sporocyst, redia, and immature cercaria) are obtained. The 

 method has the disadvantage that the mollusc is necessarily killed 

 in the course of the dissection and cannot be used for further work. 

 The technique is simple though tedious when large numbers are 

 involved. The shell, when hard and calcareous as in Cleopatra and 

 Melania, is crushed with lion forceps, placed on a shde in a drop of 

 water or weak formalin and the mass then torn apart with two 

 dissecting needles. With the thin-shelled forms like Planorbis it 

 suffices to fix the shell by piercing the central whorl with one 



