INTRODUCTION. V 



PALPOCIL. A rigid, hair-like process, occurring on the 

 tentacles of some of the Hydroida, and probably an organ 

 of touch. 



THE HYDROID COLONY. 



In a few cases the adult Hydroid consists of a single 

 polypite ; but generally many are organically united 

 and form a colony or community. Associated life is the 

 rule, and solitary existence the rare exception. Amongst 

 the animals that compose the present order gemmation is 

 universal, and by successive buddings the complex plant - 

 like structure is rapidly evolved from the primary polypite, 

 which is the product of the egg. Even in Hydra, which is 

 a solitary being, the vegetative power is active ; but the 

 buds, which are produced in profusion, are thrown off 

 instead of being retained in permanent connexion with the 

 parent stock. 



The Hydroid colony, which may include its thousands 

 of polypites, as well as a large company of reproductive 

 zooids, is the result, like the tree, of a continuous process 

 of budding, and, whatever its extent, has originated in 

 a single polypite. The analogies, indeed, between zoo- 

 phyte-life and plant-life are numerous and striking ; and 

 we shall best illustrate and explain many points in the 

 history of the Hydroida by a reference to the facts of the 

 vegetable world. 



Every Hydroid colony consists of two parts (which may 

 be considered separately) the coenosarc or common con- 

 necting substance, and the zooids held in organic union by 

 it, which discharge different functions in the service of the 

 commonwealth. 



The coenosarc is a fleshy tube (a thread of animal sub- 

 stance hollowed out in the centre) which now appears as 



