THE METAZOA — DIVISION OF LABOR 203 



Reproduction. — The hydra reproduces itself either liy budding 

 or by the production of new animals by meuus of i^ggs and sperms, 

 sexually. The bud appears on the body as a little knob, sometimes 

 more than one coming out on the same hydra. At first the bud is 

 part of the parent animal, the body cavity extending into it. After 

 a short time (usually a few days) the young hydra separates from 

 the old one and begins life anew. This is asexual reproduction. 



The hydra also reproduces by eggs and sperms. These sperms 

 are collected in little groups which usuallj- appear near the free 

 end of the animal, the egg cells developing near the base of the 

 same hydra. Both eggs and sperms grow from the outer layer 

 of the animal. The sperms, when ripe, are set free in the water; 

 one of them unites with an egg, which is usually still attached to 

 the body of the hydra, and development begins which results in 

 the growth of a new hydra in a new locality. 



The stages passed through in development resemble closely 

 those already described on page 200, and it would not be hard to 

 imagine the gastrula stage, turned upside down with a ciicle of 

 tentacles a t the open end. Our gastrula would then be a hydra. 



Division of Labor. — If we compare the amoeba and the para- 

 moecium, we find the latter a more complex organism than the 

 former. An amceba may take in food through any part of the 

 body ; the paramoecium has a definite gullet ; the amceba may use 

 any part of the body for locomotion ; the paramoecium has definite 

 parts of the cell, the cilia, fitted for this work. Since the structure 

 of the paramoecium is more complex, we say that it is a " higher " 

 animal. In the vorticella, a still more complex cell, part of the 

 cell has grown out like a stalk, has become contractile, and acts and 

 looks like muscle. 



As we look higher in the scale of life, we invariably find that 

 certain parts of a plant or animal are set apart to do certain work, 

 and only that work. Just as in a communitj' of people, there are 

 some men who do rough manual work, others who are skilled work- 

 men, some who are shopkeepers, and still others who are profes- 

 sional men, so among plants and animals, wherever collections of 

 cells live together to form an organism, there is division of labor, 

 some cells being fitted to do one kind of work, while othtTs are 

 fitted to do work of another sort. 



