190 PRINCIPLES OF ANIMAL BIOLOGY 



be brought together. The general act of bringing the germ cells or their 

 equivalents together has often been termed copulation. Under this 

 broad interpretation of the term copulation, the simplest method is that 

 of some unicellular animals, such as Vorticella, Heteromita lens and many 

 other Protozoa where two individuals meet and their bodies completely 

 fuse. A somewhat more complex reproductive method is shown by 

 Paramecium in which two individuals meet and interchange portions of 

 their micronuclei, the portion received by each being subsequently fused 

 with the portion retained within the body. As pointed out in the pre- 

 ceding chapter the uniting animals may be apparently similar (Para- 

 mecium, Heteromita lens, and Gonium), or they may be dissimilar, as in 

 Vorticella, Eudorina and Volvox. The end result is virtually the same, 

 whether the individuals are like or unlike in size, or whether there is 

 complete fusion of the bodies of the individuals or only an interchange of 

 nuclear material. The term copulation, however, is not usually applied 

 to the Protozoa, since these simple animals do not possess germ cells, as 

 distinct from somatic cells. The word copulation is in practice used 

 only for the metazoa, in which true germ cells are formed, leaving the 

 union of Protozoa for nuclear exchange to be designated by the term con- 

 jugation, or fusion if there is complete union of their bodies. Further- 

 more, while frequently used in a general sense to designate the act of 

 bringing the germ cells together, the word copulation is more accurately 

 used when restricted to the act of introducing the male elements into 

 the body of the female in those forms which have internal fertilization. 

 The act of clasping while the eggs are being fertilized outside of the body 

 or while the male is depositing spermatozoa to be later secured by the 

 female is specifically known as a^nylexus. As will be shown later not all 

 animals have copulatory habits but when these occur they are known as 

 fusion, conjugation, amplexus or copulation depending upon the nature 

 of the union. 



Fertilization in Hermaphroditic Animals. — In metazoa special sexual 

 cells (eggs and sperms) are developed. A relatively small number of 

 forms, among them some sponges, Hydra and a few of its relatives, 

 worms and snails, are hermaphroditic (Fig. 145). Among these hermaph- 

 roditic forms, as in many sponges, the production of ova usually pre- 

 dominates in some individuals, the production of spermatozoa in others. 

 In many hermaphroditic species the male and female elements are pro- 

 duced at different times, a condition known as dichogamy. When, as in 

 most tunicates, the hermaphroditic animals are regularly female at first 

 and later male, they are said to be protogynous. Others, like the mollusk 

 Crepidula, are first male and then female and are said to be protandrous. 

 Regarding hermaphroditism in general it may be said that it is more 

 common among the invertebrates than among the vertebrates; that it is 

 the normal condition in tunicates and a few fishes; that it is occasional 



