I50 AN INTRODUCTION TO ZOOLOGY. 



velop colonies, or compound animals, formed by the de- 

 velopment of single ones successively budded off from, 

 but remaining connected with, the starting-point of the 

 first organism. Bach of the units is called a "polyp,'" 

 or "zooid," or "person;" while the whole colony is 

 called a zoophyte ; the substance which connects the 

 successive units with one another is called the cceno- 

 sarc, or cosnenchyma, i.e., common flesh, or common sub- 

 stance. Corals are the most familiar instance of this 

 kind of development, which occurs in all the chief 

 divisions of the Coelenterata. Somewhat similar col- 

 onies occur in some Ascidiaus (Tunicafa), and in the 

 Bryozoa, or Moss-corals (Polyzoa) (pp. 231, 237). It 

 will easily be seen that a colony of this kind presents an 

 alternation between sexual and asexual reproduction, 

 the first individual being the product of a true egg, or 

 specialized female cell fertilized by a specialized male 

 cell, while the rest of the colony are derived from this 

 first individual by a process of division. The indivi- 

 duals thus produced bear sexual organs, just the same 

 as those produced in the ordinary way; and thus we 

 have an alternation of reproductive processes. 



Alternation of Generations. The above de- 

 scribes the state of things that exists in the colonial 

 Anthozoa, but when this alternation of reproductive 

 processes is accompanied by a specialization in form of 

 the polyps destined to bear the reproductive organs, 

 the animal is described as presenting " alternation of 

 generations," or metagenesis. This is what happens 

 frequently in the Hydrozoa (see p. 158). When this is 



