INTRODUCTION. xliii 



When liberated, it matures and disperses the generative 

 elements, and, having thus fulfilled its function, perishes. 



In other cases the gonozooids never become free, but, 

 like the alimentary polypites, remain in permanent con- 

 nexion with the colony. In this condition they exhibit 

 many diversities, and constitute a series of transition forms 

 leading up to the highest, in which the provision for a free 

 and locomotive existence is complete. 



The embryo of the Hydroida is all but universally a 

 ciliated body, the analogue of the winged seed of the 

 plant, which diffuses the species. 



RATE OF GROWTH. PHOSPHORESCENCE. 



All the evidence we possess on the point seems to show 



that the development of the fixed Hydroida proceeds 



rapidly. Timber immersed in the sea is soon found to be 



covered with a luxuriant growth of zoophyte. Mr. Couch 



considers it probable that a large specimen of Sertularella 



polyzonias may be formed, under favourable circumstances, 



in fourteen days. At Rio Janeiro a Eudendrium, allied to 



E. rameum, has been observed to cover the bottom of a 



boat in fifteen days. Stimpson mentions that, on the 



hooker which he used for dredging at Grand Manaii, 



an Obelia had reached the height of an inch in less than a 



month after the bottom of the vessel had been scraped 



clean ; and Van Beneden speaks of the great rapidity with 



which Tubularia coronata is developed along the coast of 



Belgium*. 



* The following illustration of the enormous rate at which some of the 

 Hydroids multiply is from M'Cracly: "I have observed the medusas (of 

 Tubularia cristata) fully grown and casting their larva; as early as March 10th, 

 and as late as September 13th, during all which time thousands of larv;u arc 



