XVI. THE METAZOA — DIVISION OF LABOR 



Problem XXVI. ^in i ntrodivctory stiuhj of mamj-ceUcd ai%- 

 vmals. {Laboratory Mduical , Prob. .V,\7 '/.) 

 id) Development. 

 (Z>) Sponges. 

 (c) The hydra. 



id) Development of tissues and organs. 

 (e) Cormnon functions. 



Reproduction in Simple Plants. — Although there are very 

 many plants and animals so small and so simple as to be com- 

 posed of but a single cell, by far the greater part of the animal 

 and plant world is made up of individuals which are collections of 

 cells living together. 



In a simple plant like the pond scum, a string or filament of cells 

 is formed by a single cell dividing crossmse, the two cells formed 

 give rise each to two more, and eventually a long thread of cells 

 results. Such growth of cells is asexual. 



In some instances, however, a single cell was formed by the union 

 of two cells, one from each of the adjoining filaments of the plant. 

 Around this cell eventually a hard coat was formed, and the spore, 

 as it was called, was thus protected from unfavorable changes in 

 the surroundings. Later, when conditions became favorable for 

 its germination, the spore might form a new filament of pond 

 scum. 



In the seed plants, too, we found a little plant within the seed 

 which, under favorable conditions, might give rise, through the 

 rapid multiplication of the cells forming it, to a new plant. But 

 the plant within the seed first arose from two ctslls, one of which, 

 called a sperm, came from a pollen grain, the other of which, the 

 egg, was found within the embryo sac of the ovary. 



Reproduction in Simple Animals. — lu mauy-ccllcd animals, 

 as well as many-celled plants, the new animal is formed by the 



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