INTRODUCTION TO THE METAZOA 



79 



result of a division of labor among the 

 somatic cells. 



However, there is no sharp line between 

 Metazoa and Protozoa. The colonial proto- 

 zoans are composed of many cells, which, as 

 in Volvox (Fig. 22), are differentiated into 

 germ and body (somatic) cells. However, 

 the somatic cells do not show any division 

 of labor (specialization); this distinguishes 

 such complex Protozoa from the Metazoa. 

 There is a considerable number of animals 

 intermediate between Protozoa and Meta- 

 zoa, but we do not find in any of the pro- 

 tozoans the high degree of specialization 

 which results in the various types of somatic 

 tissues, such as nerves and muscles. 



THE METAZOA 



The differences between the Protozoa and 

 the more complex Metazoa are so great that 

 we will consider some of the more impor- 

 tant metazoan characteristics before study- 

 ing the various metazoan phyla. 



In our metazoan study, the following sub- 

 jects are of fundamental importance: (1) 

 the origin of germ cells, (2) the methods of 

 reproduction, (3) differentiation of somatic 

 cells and formation of tissues, (4) the asso- 

 ciation of different tissues to form organs 

 and of different organs to form systems, and 

 (5) variations in the forms of animals cor- 

 related with differences in symmetry, meta- 

 merism, and the character of the append- 

 ages. 



Differentiation of germ 

 cells and somatic cells 



The body of a true metazoan is always 

 composed of germ cells (gametes) and so- 

 matic cells. The germ cells serve for repro- 

 ductive purposes only; the somatic cells 

 form a distinct body which carries on all 

 the functions characteristic of animals, ex- 

 cept sexual reproduction. The mature germ 

 cells are either female or male. Female germ 



cells are known as eggs (ova), and male 

 germ cells as sperms (spermatozoa). When 

 the germ cells become mature, they may 

 separate from the body, giving rise to a new 

 generation, whereas the somatic cells die. 



Methods of reproduction 



Reproduction is one of the fundamental 

 properties of protoplasm. Two types may be 

 recognized: asexual and sexual. In the fol- 

 lowing paragraphs some of the common 

 methods of reproduction are listed, but varia- 

 tions of each type will be encountered in 

 the study of the metazoans. A general ac- 

 count is presented in Chapter 34. 



Asexual reproduction 



This is reproduction without sexual 

 (germ) cells, that is, without eggs or sperms. 

 The principal methods of asexual reproduc- 

 tion are fission and budding. The amoeba, 

 Paramecium, and many other protozoans re- 

 produce by binary fission. In budding, as in 

 the hydra (Fig. 50), there is usually an out- 

 growth from the parent, a bud, which sepa- 

 rates while still small and grows into an 

 adult. 



Sexual reproduction 



This type of reproduction is by means of 

 sexual (germ) cells. Usually a female cell, 

 the egg, fuses with a male cell, the sperm. 



When eggs develop normally without be- 

 ing fertilized by spermatozoa, the process is 

 known as parthenogenesis. For example, the 

 eggs of plant lice (aphids. Fig. 157, p. 258) 

 and water fleas {Daphnia, Fig. 110) may de- 

 velop normally without fusing with sperma- 

 tozoa. 



Dioecious animals are either male or fe- 

 male; each possesses only one type of repro- 

 ductive organ that gives rise to either eggs or 

 sperms. Most species of higher animals are 

 of this type. 



Monoecious ( hermaphroditic ) animals 

 are provided with both male and female re- 

 productive organs, and produce both eggs 



