CHAPTEE II. 



THE METAZOA. 



ALL animals above the Protozoa have been classed together as 

 Metazoa, and possess the following characters in common : 



There is always more than one nucleus, and the nuclei are for 

 the most part arranged with a definite relation to the functional 

 tissues. 



Conjugation always takes place, but the structure is so complex 

 that conjugation between the ordinary individuals of the species 

 is impossible. Consequently special individuals the gametes are 

 produced for the purpose of conjugating. These individuals, which 

 have a very similar form throughout the group, are simple in 

 structure and unicellular in character ; there are always two kinds 

 of them in every species, called respectively ova and spermatozoa. 

 They arise by a process of unequal fission from their parent, and 

 may both be produced by one individual or by different individuals. 

 When they are both produced by the same individual, that indi- 

 vidual is said to be hermaphrodite. When they are produced by 

 different individuals, that parent which produces the ova is called 

 the female, while that which produces the spermatozoa is called the 

 male ; and the individuals are said to be unisexual and the species 

 dioecious. The conjugating individuals, or gametes, produced by 

 the male never have the power of assuming the ordinary form of 

 the species, and though they have, as a rule, the poAver of inde- 

 pendent locomotion, soon die unless placed in the most favourable 

 circumstances. The gametes produced by the female, on the other 

 hand, Avhile they are Avithout the power of locomotion and have a 

 rather greater power of independent life, are in rare cases capable 

 of becoming more complex in structure, and of assuming the form 

 of the adult. To females Avhich produce such ova the term partlieno- 

 yenetic is applied. In the vast majority of cases, however, the ovum 

 has not the poAver of changing its form and of developing into 



