WATSON : DEFICIENCIES IN THE VERTIGININ^. 277 



Boycott has pointed out/ but also by the fact that, while 

 spermatozoa are present in considerable numbers in the 

 bermapbrodite gland in these forms, irrespective of whether the 

 animal has a penis or not, they do not occur in the enormous 

 quantities that one commonly finds in the genital glands of other 

 snails. This is probably due to the small size of the gland, which 

 does not leave room for a very large number, most of the space 

 being occupied by the ova. In this connexion it is interesting to note 

 that, of the species examined, the two in which the swelling of the 

 hermaphrodite duct that serves as a vesicula seminalis is largest, 

 namely, V. antivertigo and V. pygmcea, are the only species of Vertigo 

 in which all the specimens examined possess a penis. 



Dr. Boycott has also pointed out that it would be specially 

 advantageous to very small snails to dispense with any superfluous 

 organs, in order to leave greater room for the more essential 

 structures. 2 The complex organs possessed by a snail are in most 

 cases built up of a very large number of cells, and the size of the 

 individual cells cannot be reduced indefinitely^; therefore a stage 

 must be reached when some of the organs can scarcely become any 

 smaller. Accordingly, in a minute snail the organs are likely to become 

 unduly crowded, and it will then be an advantage to dispense with any 

 that are imnecessary in order to leave room for the efficient working 

 of those that are more essential. If the male organs can be dispensed 

 with, more room will be left for the adjacent buccal mass and central 

 nervous system, very complex and necessary structures.* And there 

 will also be more room for the passage of the eggs down the female 

 ducts ; for the eggs cannot be reduced below a certain size if they are 

 to contain a sufficient store of food material to carry the young snail 

 through the whole of its development, until it is hatched in a form in 

 which it is able to feed itself. 



To this argument it might perhaps be objected that some of the 

 species with male organs, Vertigo pygmcea for example, are even 

 smaller than some of those that are frequently without a penis. 



1 Proc. Malac. Soc, vol. xii, 1917, p. 225. It may be observed that the 

 three species in which no specimens without male organs have yet been found 

 are those that are the most widely distributed throughout the British Isles. 

 On the other hand, some of the species that seem to be usually without male 

 organs are sometimes found in considerable numbers in the small areas in 

 which they occur ; and in such instances it may be doubted whether the 

 chances of two individuals meeting each .other are less than in the case 

 of a few of the larger, but less gregarious, forms belonging to other groups 

 which are not known ever to lack a penis. Moreover, the species of Vallonia, 

 which seldom possess male organs, are by no means rare snails. 



2 Journ. of Conch., vol. xv, 1917, p. 177. 



* See D'Arcy Wentworth Thompson, On Growth and Form, 1917, pp. 34-8, 

 on the factors that limit reduction in the size of cells. 



* It is even conceivable that the pressure of these organs at an early and 

 critical stage in the development of the genital ducts might possibly be a 

 direct cause of the suppression of the male organs. 



