128 Life and Death, Heredity and Evolution 



neither with one another nor with any males that may be 

 present. So the ordinary individual is neither male nor 

 female; it is apparently non-sexual or neutral. After it 

 has divided to produce male and female, the female resembles 

 externally the neutral individual, but differs from it in 

 that it may mate. If it has no opportunity to mate, it 

 may return to the neutral condition, and may then again 

 divide to produce males and females. These facts have 

 been worked out in Opercularia by Enriques (1907). In 

 some other Vorticellidae a single ordinary individual may 

 divide into four or eight males. It is asserted that some- 

 times the small motile "males" mate together; and that 

 similarly sometimes two fixed "females" mate; but these 

 things are uncertain. There is still much to be learned 

 as to sex diversity in this most interesting group of the 

 Vorticellida. 



In some infusoria the two individuals that mate are alike 

 before mating, but become changed and diverse during the 

 mating process. This is inevitable in some infusoria, since 

 the organisms are unsymmetrical, and if their mouth sur- 

 faces are to be brought together, either the two must take 

 different positions, or one or both must become altered 

 in structure. This will be evident from Figure 38. 



Enriques (1908) studied this matter thoroughly in 

 Chilodon. In this animal the mouth lies to the left on the 

 ventral surface, so that when the two individuals are placed 

 side by side, the mouth of the right-hand individual lies at 

 the surface of contact, but the mouth of the left-hand in- 

 dividual is directed away from the surface of contact (Fig- 

 ure 38). But in the early stages of the process of mating, 

 the mouth of this left-hand individual moves along the ventral 

 surface over to the right, so as to meet the mouth of the 

 other individual which does not change its position (Figure 

 39). The anterior end of the left individual likewise be- 



