140 ISABKLI.A M. DRUMMOND. 



2. Tliere is some evidence, botli from comparative anatomy 

 and embryoloi^y, for believing that the oesophagus has 

 undergone an actual twist. 



3. Monstrosities which retain the ]:)alleal complex in a 

 ventral position show a tendency to form an exogastric 

 coil. 



The innervation of the mantle was shown to be equally 

 difficult to explain on either hypothesis. 



Also, against the first view was urged the insufficiency of 

 the evidence upon which Butschli bases his conclusions with 

 regard to zones of unequal growth. 



With regard to the phylogenetic cause of the vertical 

 twist, embryology can only give negative evidence ; while in 

 considering the ontogenetic cause we are thrown back upon 

 unsolved problems of heredity, and must confess our 

 ignorance. 



In conclusion, I wish to offer most hearty thanks to 

 Professor Weldon, not only for having ])laced freely at my 

 disposal all the resources of the laboratory, but also for most 

 kind personal aid at all stiiges of the work. My thanks are 

 also due to Mr. liichard Evans for much help in the 

 technique. 



List of Works referred to in the Text. 



1. Amauduut, a. — "La Faitic anlerieure dii Tube digcblil clitz Its 



Mollusques Gasteropodes," ' Ann. des Sci. Nat. Z()oI.'(8), vii. 



2. BoUTAN, L. — " Lh Cause principale de rAi>yniet.iic dcs Mollub(jUc;s 



Gaslcropodes," 'Aicli.de Zool. exp.' (3), vii. 



3. HouviiiR, E. L. — " Systcine uerveuse, Morpliolo^ie ct ClassKicalion des 



Gasteropodes Prosobiaiiclies," 'Ann. des Sci. Nat. Zool.' (7), iii. 



4. i)€TsciiLl, 0. — " J3emerkun£^('U ueber die walirsclieinliehe lleiltituDg 



der Asyniuietrie der Gast.ropoden," ' AIoipli. Jaiiib.,' xii. 



5. Kklangeu, von K. — " Zur Eiitwickhuigsiresciiiclite voii raludiuu 



viviparu," ' Morpli. Jalirb.,' xvii. 



6. Eklangek, von ]{. — "Zur Eiitwicklungsgescliielitc von I'aludina 



vivipara (vorliiufige Mitllieilungen)," 'Zool. Aiiz.,' xiv. 



7. EuLANGER, VOiN 11. - " Ou t.Jie Failed Ne|iliiiilia of Prosobianchs," 



' U,. Jouiii. Micr. Sci.,' x.Nxiii. 



