ORGANIC DEPENDENCE AND DISEASE 53 



eom. There are long tubular passages between the coral- 

 lites in early-growth stages which have not been described 

 in the structure of this coral genus but undoubtedly belong 

 to it. In sections these may be confounded with worm tubes, 

 but in etched specimens, such as have here principally 

 served for illustration, their real nature seems to be clear. 



In this interesting combination there is still another 

 member — a small silicious sponge. It has come to my 

 notice several times. The one here figured was taken from 

 the empty tube of the worm, but whether that is its usual 

 position or whether it may seat itself in one of the coral 

 calyces or whether indeed it is a usual member of the con- 

 sociation cannot be regarded as clearly established. Its im- 

 portance is not to be magnified; such little organisms are 

 easily entangled in growing corals and must be expected in 

 the fossil state. 



Some illustrations are here given which show how readily 

 the dead parts of these organisms become encrusted with 

 serpulid worms. Figure 29 is the surface of a part of a 

 dead Loxonema to which a Pleurodictyum had grown, and 

 figure 32 shows the inside of an old tube of the commensal 

 worm (which is known as Hicetes innexus), itself incrusted 

 with minute worm tubes. 



Interesting as is this instance of commensalism, its most 

 extraordinary feature is the evidence of selection by the 

 larval coral, of the body which serves as the base on which 

 it is to grow. It is stated above that a very evident ma- 

 jority of the colonies of this coral Pleurodictyum, as it oc- 

 curs in the Hamilton shales, are attached to an organic 

 object and that this organic base in apparently a very 

 large majority of the cases is a shell of Loxonema hamil- 

 toniae. Occasionally the shell may be a Pleurotomaria of 

 one or another species. On the other hand the Rhenish 

 Pleurodictyum prohlematicum fixes itself by decided ma- 

 jority to the brachiopod Chonetes sarcinulatus Schlotheim. 

 I have examined a considerable number of specimens 



