17 6 SYSTEMATIC ZOOLOGY FOR TEACHERS. 



BRANCH METAZOA. 



Many-celled Animals. 



Although the animals included in this group begin life 

 as a simple, single cell, (ovum) this single cell divides into 

 many cells when placed under the proper conditions for de- 

 velopment. These divisions, in most cases, no longer remain 

 simple in form, but become changed (differentiated) into va- 

 ried forms, like nerve cells, muscle cells, bone cells, etc. The , 

 cells thus changed assume varied functions to meet the wants 

 of the animal of which they form a part. Propagation is ac- 

 complished in three ways; primarilly, however, it is always by 

 eggs, and these eggs are always fertilized, by the accession of 

 another cell in the form of a spermatozoon. The secondary 

 method, but one which is found in the lowest invertebrate 

 forms of animal life only, is division, much as 'we have 

 found it in the protozoa, and this method is accomplished 

 without special fertilization. A third method, less common 

 even than division, is budding. This is also accomplished with- 

 out special fertilization. The first method of propagation is 

 known as the sexual method ; the second and third methods 

 as' asexual. 



The animals included in the present group from the lowest, to the high- 

 est are so very devious in character that but little can be said which will ap- 

 ply to all of the forms. As they all begin life, however, as a pimple, single 

 cell, known as the ovum or egg, we will trace this cell through jts preliminary 

 stages, which from the lowest form, the sponge, to the highest, man, are essen- 

 tially the same. 



i 



Egg Division or Segmentation. 



,y ,-.;•• 



The ovum or egg is the product of the female, either as an individual 

 or as a portion of an animal. 



When the ovum becomes sufficiently developed to receive the fertilizing 

 element, it is said to be ripe. The fully developed speimatczcca, although 



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