Descriptive Biology 103 



occurs, especially in the parasitic forms. Here the production 

 of numerous spores undoubtedly increases the distribution of 

 the organism, but the meaning of the sexual phase in the life 

 cycle is as obscure as is that of sexual reproduction among 

 living things in general. The whole problem of reproduction, 

 in all its manifold phases is one of the most perplexing which 

 biology has to solve. 



In the majority of animals the sexes are distinct, but in 

 some, notably flat worms, annelids, some molluscs, etc., they 

 are found in the same individual, which is then known as an 

 hermaphrodite from Hermaphroditus, the Son of Hermes and 

 Aphrodite, who became joined in one body with the nymph 

 Salmacis. How comes it that in some animals we find distinct 

 sexes, while in others, not distantly related to them, they are 

 united ? Here again we are face to face with one of Nature 's 

 inscrutable mysteries. A possible clue is found however in 

 the fact that in animals of separate sex, the early stages of 

 the sex organs may be apparently "indifferent," that is, 

 neither male nor female, becoming differentiated into either 

 sex as development proceeds. Even though sex may be, as 

 we shall see later, predetermined in the fertilized egg, never- 

 theless all the essential parts of both male and female may 

 develop in the embryo. Furthermore, there are many in- 

 stances, some of which will be mentioned in another chapter, 

 of so-called "sex intergrades" where the animal or plant may 

 be predominantly of one sex and yet show some of the 

 characteristics of the opposite sex; and further, when the sex 

 glands cease to function, either from disease, removal or old 

 age, certain characters of the opposite sex may appear, as 

 noted in a later chapter. 



Apparently then the hermaphroditic condition is primitive 

 and the bisexual one derived, through suppression, but not 

 loss in either sex of the characters peculiar to the opposite sex. 



But why this differentiation has occurred and the advantage 

 thereof we do not know. 



Passing from the unicellular to the multicellular animals 

 or Metazoa we find one of the simplest of the latter in a 

 little creature which strangely enough bears the name of the 

 many-headed monster, Hydra. Its body consists of two layers 

 of cells and contains a primitive digestive cavity with a 

 mouth, which is surrounded by several tentacles, giving the 

 animal its fanciful resemblance to the horrid monster of 

 fabled story. The space between the layers contains an almost 

 negligible jelly and the muscle processes of cells lying in the 

 two layers. 



From Hydra the next step in advance is very uncertain. 

 The reader may best be spared the mental contortions neces- 



