Notes on the Development of Paludina vivipara, with special reference to the Urinogenital Organs and Theories of Gasteropod Torsion

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Abstract
To sum up, then, theories of Gasteropod torsion may be divided into two classes: a. Those which view the present position of the palleal complex as due to a forward movement along the right side of the body, which resulted from greater growth of the left side of the body than of the right. h. Those which view the present position of the palleal complex as due to a ventral flexion followed by a vertical rotation of the whole visceral hump upon the head. The evidence for the second of these views seems greater than that for the first, in that-- 1. A vertical displacement through 180° of all the organs contained in the visceral hump takes place in the course of ontogeny. 2. There is some evidence, both from comparative anatomy and embryology, for believing that the oesophagus has undergone an actual twist. 3. Monstrosities which retain the palleal 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 Bütschli 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 placed freely at my disposal all the resources of the laboratory, but also for most kind personal aid at all stages of the work. My thanks are also due to Mr. Richard Evans for much help in the technique.

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