FOURTH REPORT OF THE DIRECTOR ICpJ 1 57 



they have been buried in the later accumulations of stereom. 

 There are long tubular passages between the corallites in earh' 

 growth stages which have not been described in the structure of 

 this coral genus and in sections these are confounded with worm 

 tubes but in etched specimens such as have here principally served 

 for illustration, their nature is clear. 



In this interesting combination there is still another member — a 

 small calcareous sponge. It has come to my notice several times. 

 The one here figured was taken from the 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 m.ember of 

 the consociation can not clearly be regarded as established. 



I have given (pi. 4) some illustrations which show how readily 

 the dead parts of these organisms become incrusted with serpulid 

 worms. Figure 8 is the surface of a part of a dead Loxonema to 

 which a Pleurodictyum had grown and figure 7 shows the inside 

 of an old tube of the commensal worm Hicetes innexus, 

 itself incrusted with minute worm tubes. 



Interesting as is this instance of commensalism, its most extra- 

 ordinary feature is the amazing evidence of selection by the larval 

 coral of the body to serve as the base on which it is to grow. I have 

 stated above that a ver}^ evident majority of the colonies of this coral 

 Pleurodictyum as it occurs in the Hamilton shales are attached to 

 an organic object and that this organic base in approximately 80 

 per cent of the cases is a shell of Loxonema hamiltoniae. 

 Occasionally the shell may be a Pleurotomaria of one or another 

 species. I have no record of its being any other than one of these 

 gastropods. On the other hand the German Pleurodictyum 

 problematicum fixes itself by decided preference to the brachi- 

 opod Chonetes sarcinulatus Schlotheim. I have ex- 

 amined a considerable number of specimens of this Coblentzian 

 species but have seen no other shell used for attachment nor have I 

 found record of any other. Though I can not use percentages with 

 reference to the frequency of this occurrence, this palpable fact remains 

 that as between these two closely allied if hot identical corals, one 

 selects a gastropod, the other a brachiopod as its base of attachment. 

 Emphasis is to be put on the word "selects" for among the brilliant 

 examples of selective adaptation none could be more striking than 

 this. The floor of the New York ocean was covered with Chonetes 

 and of the German ocean with gastropods during the life of this 

 coral. Were either wanting in the other fauna, hundreds of other 

 species of organisms lined the sea bottom. The coral was not de- 

 prived of its choice. 



