ORGANIC DEPENDENCE AND DISEASE 
47 
uals and it is safe to say that the worm is present in the 
majority of examples. It is usually easy to determine its 
presence on inspection of the tentacular surface of the coral 
by the contrast between its round tubes and the angular 
coral cells. All the specimens here figured to show the con- 
volutions of the worm have been drawn from actual prep- 
arations. 
The history of the combination in P. styloporum is as 
follows : At the close of the free-swimming larval stage the 
coral, in fully eight cases out of ten, selected and attached 
itself to a dead or living shell of the common gastropod 
Loxonema hamiltoniae. 1 Directly upon fixation or even 
actually contemporaneous with it was the attachment of the 
larval worm upon the gastropod and alongside the incipi- 
i The selective attachment of such lens-shaped coralloid stocks seems to have 
acquired directiveness with the progress of time. At any rate we have a sug- 
gestive intimation of this in the very common Chaetetes ly coper don ( Prasopora 
simulatrix ) in the Trenton limestone of the Ordovician, which is a stony coral 
Fig. 21. Basal surface of the solid bryozoan colony, Prasopora selwyni, which 
has attached itself to the brachiopod Plectambonites. Trenton limestone 
(Ordovician), Ottawa. 
of quite the same shape and habit of growth as these Pleurodictya. This is 
found attached sometimes to brachiopod (especially Plectambonites sericea ) 
and as often to gastropod shells which were the abundant exuviae of the sea 
bottom. More often perhaps it is fastened to some casual stone or other hard 
object, but among all of which I have taken note there seems to have been no 
obvious preference by majority. 
