ALCYONIUM. 107 



settles down on a rock or shell at the bottom, loses its cilia 

 and assumes the characters of a single Alcyonarian polyp. 

 The youngest stage that the author has seen after fixation 

 is shown on Plate III., fig. 24. In this case the primary 

 polyp has already formed one secondary polyp by gem- 

 mation. 



Physiology. 



When the polyps of an Alcyonium are fully expanded 

 they are in a state of physiological activity, the muscles 

 are constantly contracting, the cilia vibrating, and other 

 functions of the body being performed. This functional 

 activity does not go on continuously, but at times the 

 anthocodise are all retracted, and a period of rest super- 

 venes. There can be little doubt that the period of rest 

 occurs rhythmically, the rhythm corresponding not with 

 the light and darkness of day and night, but with the 

 high and low tides. As a result of some experiments that 

 were made a few years ago, it seems probable that 

 Alcyonium rests regularly at every low tide, that is to say, 

 twice in ever}' day and night, but owing to the unsatis- 

 factory conditions appertaining to life in a sea-water 

 aquarium, it is not possible to state how long the periods 

 of rest last in natural healthy surroundings. 



Circulation. — It is certain that in such a fleshy mass 

 as a colony of Alcyonium presents, a free circulation of a 

 liquid throughout the whole system is absolutely necessary 

 for the respiration of the tissues. In the absence of any 

 rhythmically contractile organ which could be called a 

 heart, how is this circulation maintained ? The answer 

 is, entirely by ciliary action. When the polyps are 

 expanded, a current of water is produced by the constant 

 vibration of the long cilia of the siphonoglyph, which 

 flows from the mouth downwards into the coelenteric 

 cavities. It is probable that some of the fresh sea-water 



