xiii ALCYONARIA FOOD 339 



proceeds thence in an undulating wave to the extremity of the 

 rachis, but never in the opposite direction ; it is only the parts 

 at and above the point of stimulation that show phosphorescence, 

 the light is emitted for a longer time from the point of stimula- 

 tion than from the other luminous parts ; detached portions may 

 show phosphorescence. When plunged in fresh water, the Pen- 

 natula scatters sparks about in all directions — a most beautiful 

 sight." 



Panceri was of opinion that the mesenteric filaments were 

 the organs of phosphorescence, but the whole question of the 

 cause and localisation of the light in these colonies requires 

 further investigation. 



Food. — Very little is known about the food of Alcyonaria, 

 but it is very probable that it consists entirely of minute larvae 

 and other living organisms. When the coelenteric cavities of 

 preserved Alcyonaria are examined, food is very rarely found in 

 them, although fragments of Crustacean appendages have occa- 

 sionally been seen in the neighbourhood of the mesenteric 

 filaments. Experimenting upon Alcyonium digitatum, Miss 

 Pratt ^ has found that the zooids seize and swallow various small 

 organisms of a surface -net gathering, and that they will also 

 swallow finely minced fragments of the muscle of fish, but that 

 they reject many kinds of fish ova. In many tropical and some 

 extra - tropical species the superficial canal systems and the 

 inter-mesenterial spaces of the zooids contain a large number of 

 Zooxanthellae, and their presence seems to be associated in some 

 cases with a decided degeneration of the digestive organs. It 

 has been suggested that these symbiotic " Algae " prepare food 

 materials after the manner of plants, and that these are absorbed 

 by the hosts, but it appears improbable that in any case this 

 source of food supply is sufftcient. It must probably be supple- 

 mented in some degree by food obtained by the mouth, and 

 digested in the coelenteric cavity. 



The question whether the Alcyonaria can form an important 

 part of the dietary of fish or other carnivorous animals may be 

 economically important. Fragments of the Pennatulid Virgularia 

 have been found in the stomachs of cod and other fish, but with 

 this exception there is no evidence that any genus is systematically 

 or even occasionally preyed upon by any animal. With a very 

 1 Quart. Journ. Micr. Sci. xlix. 1905, p. 327. 



