MORPHOLOGY Of 1 ANIMALS. 333 



In many Protozoa something similar occurs. Here we 

 find a union of different individuals, and as each protozoan 

 is a single cell, this union of individuals is comparable, to 

 a certain extent, to the union of egg-cell and sperm-cell. 

 With the Protozoa, however, after this union (conjugation) 

 the individuals separate and each divides, thus producing 

 new individuals (cells), which differ from the cells produced 

 by the division of the egg in that they never arrange 

 themselves into layers, but each forms a distinct individual 

 like the parent. 



Besides this sexual reproduction many animals possess 

 the power of reproducing asexually. In these cases the 

 animal L may divide into two, or a small portion may pro- 

 trude as a bud which will eventually produce an individual 

 more or less like the parent. This asexual reproduction is 

 very common among the ccelenterates, but it may also oc- 

 cur among the lower worms (p. 243), the Polyzoa, the 

 tunicates, etc. 



In many instances this asexual reproduction does not re- 

 sult in the formation of distinct and separate animals, but 

 buds and parents may remain somewhat intimately con- 

 nected with each other, the result being the formation of 

 what are known as colonies, of which Pennaria may be 

 taken as a type. Here we are met with a difficulty in the 

 use of terms. We have spoken heretofore of individuals ; 

 but is each zooid in a colony of Pennaria an individual, or 

 is the colony itself to be so regarded, the zooids being re- 

 garded as organs ? 



In many cases this reproduction by budding results in 

 the formation of parts very different from each other. Thus 

 in the hydroid (Fig. 147) abundant on shells inhabited by 

 hermit-crabs, the colony consists of three different kinds of 

 hydranths: (1) the feeding hydranths (/) which take 



