IO JOURNAL OF CONCHOLOGY, VOL. 9, NO. I, JAN., 1898. 



details of the bodies of these animals. The questions that naturally 

 occur to the conchologist are these— "Why is it that some gastropods 

 such as the limpet have shells which are bluntly conical, while others 

 such as the periwinkle, living under almost precisely similar conditions, 

 have a shell twisted up into a spiral form ? " 



Anatomical study of the periwinkle shows that it is not only the 

 shell, and the mantle which secretes the shell, that are twisted up into 

 a spiral, but that there is also a distinct twist in certain parts of the 

 nervous system and in other organs of the body. 



Let us consider for a moment the structure of the nervous system 

 of one of these twisted gastropods, such as that of Paludina. We find 

 that the cerebral ganglia, the buccal ganglia, and the pedal ganglia 

 are perfectly symmetrical, those on the right side being equal to and 

 similar to those on the left. The twist is seen in a nerve loop pro- 

 vided with three ganglionic enlargements, which supplies principally 

 the alimentary tract and some of the other viscera. 



Now, if we study the development of these molluscs, we find that 

 the twist is accompanied by, if it is not due to, the imperfect develop- 

 ment of some of the most important organs of one side of the body, 

 usually the right side, and that this feature is recognisable at a very 

 early stage in development, i.e., according to the recent researches of 

 Crampton during the segmentation of the egg. 1 



It is pertectly clear, therefore, that the spiral shell of the gastro- 

 pods is a sign of very far-reaching anatomical peculiarities which, from 

 the early stage at which they appear in the embryo, must have made 

 a very ancient appearance in the evolution of the class. 



Now, if we turn our attention to other gastropods, such as 

 Aplysia, Umbrella, and many of the pteropods, we find that the shell 

 is not spiral in the adult, nor is the visceral loop ot the nervous system 

 twisted into the form of the figure 8, as it is in the whelk. On the 

 other hand, we find that some of the organs of the body, such as the 

 kidney, etc., are single, as in the whelks and periwinkles, indicating a 

 suppression of the organs of one side of the body as in those forms. 



The difference between these two groups is expressed in Spengel's 

 terms : Streptoneura and Euthyneura, the former including those 

 gastropods with a twisted visceral nerve loop, and the latter those 

 with an untwisted nerve loop. There is no doubt that when mala- 

 cologists came to realize the importance of Spengel's anatomical 

 discoveries, the opinion was very generally held that the Streptoneura 

 and the Euthyneura represented two branches from a common an- 

 cestral stock, which had diverged from one another in the extent to 



i. Crampton, Experimental Studies on Gasteropod Development, Arch. Entwickmech, 

 vol. 3, p. 1-19, 4 pi., 1896. 



