INTRODUCTION. XXV11 



development and perfect form on leaving the parent stock; 

 in a large proportion of cases it undergoes very con- 

 siderable change subsequently. The form of the umbrella 

 may alter, and the marginal tentacles and other bodies 

 and even the radiating canals increase greatly in number ; 

 while the manubrium may become much elongated, or 

 develope additional oral appendages *. The early and 

 mature states are often so dissimilar as to have been re- 

 ferred to different species ; and as there is seldom the 

 opportunity of observing the whole course of development, 

 the varying phases of the sexual zooid are a source of 

 much perplexity to the systematist f. 



Gemmation is not confined to the fixed portions of the 

 Hydroid colony ; it also enters into the history of the free 

 and locomotive zooids. In many cases they manifest the 

 vegetative tendencies of their tribe, and multiply rapidly 

 by budding. Gemmation seems usually to take place 

 when the true reproductive function is in abeyance. Thus 

 in the spring the gonozooid of Clavatella developes buds 

 on the margin of the body between each pair of tentacles, 

 which are cast off at a certain stage of growth ; while 

 later on in the year the vegetative activity ceases, and 

 reproduction by ova and spermatozoa takes its place. 

 These buds, which are analogous to those produced in 

 such profusion by the Hydra, bear an exact resemblance, 

 when mature, to the zooid that originated them. In other 

 cases they spring from the manubrium, or from the bul- 



* A. Agassiz has pointed out that the tentacles are developed in a certain 

 fixed order, and has given the formula of development for many species. 

 (Proc. Boston Nat. Hist. Soc. vol. is., August 1862.) 



t A good illustration of the changes which the detached zooid may undergo 

 before reaching maturity, and of the complexity of structure which it may 

 finally attain, is afforded by the genera Bouff(dut'i[li and 7,</yt>ducf>/[a. 



