406 GENERAL ZOOLOGY. 



tion may protrude as a bud which will eventually produce 

 an individual more or less like the parent (gemmation). 

 This asexual reproduction is very common among the 

 Coelenterates, but it may also occur among the lower 

 worms (p. 179), the Polyzoa, the tunicates, etc. 



In many instances this asexual reproduction does not 

 result in the formation of distinct and separate animals, 

 but buds and parents may remain somewhat intimately 

 connected 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 in- 

 dividuals; but is each hydranth in a colony of Pennaria 

 an individual, or is the colony itself to be so regarded, the 

 hydranths being regarded 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. 175) abundant on shells in- 

 habited by hermit-crabs, the colony consists of three 

 different kinds of hydranths: (1) the feeding hydranths (/) 

 which take nourishment for the whole colony; (2) the 

 protective hydranths (p) which lack mouths, but which 

 are richly provided with nettle-cells; and (3) the repro- 

 ductive hydranths (r), the sole function of which is the 

 reproduction of the species. In the Siphonophores this 

 differentiation is carried still farther (p. 168), for here 

 seven different forms may be developed, and here we 

 notice a marked fact in colonial conditions. The more 

 different the members of the colony become the more it 

 conveys the impression of being a single animal instead of 

 an aggregate. 



When there are but two different forms in the history 

 of the species it is called dimorphic (from the Greek mean- 



