THE DRVEr.OPMENT OF PALUPINA VIVIPARA. 129 



consequent rearrangement of tlie organs within the visceral 

 hump. These two processes are, of course, quite distinct. 



It has ah'eady been necessary to touch upon the subject of 

 the torsion of the oesophagus. Amaudrut, as noticed above, 

 has worked out with great care the twisting of the anterior 

 aorta, the salivary glands, etc., about the oesophagus, and has 

 come to the conclusion that this is due to an actual torsion of 

 the region of the body between the head and visceral hump, 

 but hitherto, so far as I know, no embryological evidence ha 

 come to hand. I have already described (p. 119) the curious 

 compression of the oesophagus, and the manner in which, in 

 transverse sections, the long axis changes its direction in 

 Paludina embryos. This is not direct proof, but it is difficult 

 to find any other explanation of the occurrence, except that it 

 is due to the oesophagus being forced to undergo an actual 

 twist. In quite old embryos, and in the adult, the oesophagus 

 is no longer compressed, and this appearance is quite lost. 



Now, granted that such a twist does take place, it follows, 

 as Pelseneer points out, that originally i. e., in the untwisted 

 forms, the shell, if coiled, must have been coiled exogas- 

 trically. It is exceedingly difficult to get any direct evidence 

 upon this point, for, as Plate remarks, we cannot rely on the shell 

 of any of the primitive Prosobranchs, like Fissurella, as these 

 have all undergone torsion, and, on either view, anexogastric 

 shell, if present, must be secondary ; while, on the other hand, 

 coiling does not begin to take place sufficiently early in the 

 course of development to give us clear evidence either way. 

 It is, however, worthy of remark that all the monstrosities 

 that I came across which, for some reason or other, had 

 remained untwisted, if they showed any tendency towards coil- 

 ing of the visceral hump at all, had begun to coil exogas- 

 trically. Of these the most highly developed is shown in 

 fig. M II, and has been already described. There were, 

 however, two others that showed a distinct tendency in the 

 same direction, while I did not find one with anything like 

 an endogastric coil. This fact seems to me highly significant. 



The last of these broadly marked points of difference 



VOL. 46, PART 1. NEW SllKlES. I 



