HERMAPHRODITES 185 



Hermaphroditism 



Most commonly ova and spermatozoa are produced by 

 different sexes, females and males. This is, however, far less 

 common in plants than in animals, for a considerable number 

 of animals and most plants are hermaphrodite, that is to say, 

 they have both male and female organs in the same body. 

 Common examples of these are the majority of plants — which 

 are then called monoecious — the sponges, coelenterates, leeches, 

 earth-worms, snails and some Crustacea and molluscs. 



The hermaphroditism of certain forms is a permanent 

 feature ; but in other animals it may be a matter of time : in 

 a mollusc known as the slipper-limpet, Crepidula, which is 

 so destructive to our oyster beds at Colchester, the growing 

 animal is first of all neuter, then it becomes a male, then it 

 changes into an hermaphrodite, and lastly it becomes a female. 



Similar changes take place in certain hermaphrodite Nema- 

 TODA, in the Turbellaria, and in the parasitic Crustacea 

 known as the Rhizocephala, spermatozoa developing in the 

 ovary in increasing numbers in successive generations, until 

 the animals become completely monoecious. In the Vertebrata 

 hermaphroditism is abnormal. Hermaphrodites generally pair 

 with another hermaphrodite, the spermatozoa of specimen A 

 fertilizing the ova of B, and the spermatozoa of B ferti- 

 lizing the ova of A, so that there is a cross-fertilization. 

 But there are other cases, especially among the parasites, 

 where the spermatozoa of an animal fertilize its own eggs. 

 Both flukes and tape-worms are at times self-fertilizing. 



The dramatist Sheridan has told us that even "an oyster 

 can be crossed in love"; but the physiology of the reproduc- 

 tion of that mollusc was not well understood in the days of 

 that gifted dramatist. It seems to be clear now that a young 

 oyster is a male, that after it has discharged its spermatozoa 

 it may be changed into a female at the age of one year, and 

 after it has spawned it may again revert to the male condition. 

 The changes of sex are repeated, and as far as one knows they 

 may be due to external causes, such as temperature. As 

 Stephen Paget tells us, "Oysters in their way are quite as 

 wonderful as poets, saints and men of science." 



