( 97 ) 



VI. — The Genetic Succession of Zooids in the Hydroida. By Professor 



Allman. 



(Read 16th May 1870.) 



Though most of the terms employed in the following paper have already 

 become part of the language of science, some definitions may be here given with 

 the view of rendering the subject more intelligible. 



The Zooids are the more or less individualised members of which the hydroid 

 colony is composed. 



The Hydranth is the proper nutritive zooid. 



The Blastostyle is a columnar zooid destined not for nutrition, but for the 

 origination of sexual buds. 



The Blastocheme is a medusiform zooid which gives origin to generative 

 elements, not immediately, but through the intervention of special sexual 

 buds. 



The Gonophore is the ultimate generative zooid, that which immediately 

 produces the generative elements. It may be either medusiform or sacciform. 



The Trophosome is the entire assemblage of nutritive zooids in a colony. 



The Gonosome is the entire assemblage of generative zooids in a colony. 



From all the facts which the study of the Hydroida has made apparent, we 

 may regard it as certain that however long zooidal multiplication may continue, 

 this is not sufficient for the perpetuation of the species, but that a period must 

 at last come in the life of the hydroid when by an act of true sexual reproduc- 

 tion, new individuals are produced for the indefinite extension of the species 

 through time. 



This truth finds its expression in Steenstrup's famous law of " Alternation 

 of Generations," — a law which, though not very correctly enunciated by its 

 framer, may be regarded when properly expounded as a statement of the fact, 

 that in certain animals every act of embryonal development is followed by 

 one or more acts of zooidal development, which invariably conduct us to an 

 ovum in which embryonal development followed by zooidal development again 

 occurs, and the entire series becomes thus repeated. . 



Now the various series expressing this alternation of sexual with non- 

 sexual development, exhibit among the Hydroida different degrees of complica- 

 tion, which will be more easily understood if we attempt to present them in 

 the somewhat technical shape of formulas. 



VOL. XXVI. PART I. 2C 



