THE DIGESTIVE ORGANS OF THE ALCYONARIA. 349 
attributed to the degeneration of the zooids, which appear to 
have not only lost the power of capturing living prey but also 
of killing and digesting it. 
I have already shown that zoochlorelle are most numerous 
in the genus Sclerophytum, in which the reduction of the 
digestive surface reaches its extreme limit. It therefore 
seems very possible that these algal cells indirectly contribute 
nutriment in a soluble condition to the corals they inhabit, as 
well as directly by their actual digestion. This matter is 
further discussed in the following portion of the paper devoted 
to the description of the structure and function of the zoo- 
chlorellee. 
ZOOCHLORELLZ. 
I have already described the occurrence and relative 
abundance of these algal cells in Sarcophytum, Lobo- 
phytum, and Sclerophytum (1908), and have drawn 
attention to the fact that while they usually occur in colonies 
inhabiting shallow water they are found to be fairly abundant 
in specimens from 24—34 fathoms, so that their numbers do 
not appear to be affected by bathymetric variations within 
certain limits. I have suggested that their presence in enor- 
mous numbers in the superficial tissues in certain species of 
Sclerophytum is correlated with a reduction in size of the 
tentacles and mesenterial filaments. 
The geographical distribution of zoochlorellz is interesting. 
These algal cells commonly occur in tropical Alcyonaria, but 
are usually absent in corals inhabiting the temperate and 
cold waters of British seas and the South Atlantic (Cape of 
Good Hope). Hickson (1894), however, describes them as 
being extremely abundant in species of Clavularia from 
the Victorian Coast of Australia. 
In a preceding part of the present paper I have dis- 
cussed the more or less gradual reduction of the digestive 
surface in tropical members of the Alcyonaria, and the 
accompanied increased abundance of zoochlorelle in connec- 
tion with the scarcity or absence of food in these forms. 
