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SCIENCE 



[N. S. Vol. XXXIII. No. 843 



times too closely approximates that of the 

 present without carrying with it the solu- 

 tions suggested by the soft bodies of to- 

 day. Our statement of inferences in this 

 problem must be very guarded because of 

 our being bound down only to such records 

 as the rocks have been able to retain. But 

 lest these evidences be underestimated, let 

 me insist that nature has dealt kindly in 

 the retention of the most intimate struc- 

 tures and here again, so far as skeletal re- 

 mains go, we can safely fall back on her 

 beneficence. 



Purely symbiotic mutualism is of very 

 ancient date and we find frequent evidence 

 of commensalism far back in Paleozoic 

 faunas, of the same type of combinations as 

 abound in the seas of to-day. Thus there 

 are few commoner examples of mutual 

 associations in the present sea than those 

 of the worms and corals, the worms 

 and sponges, sessile eirripeds and corals, 

 and I have elsewhere indicated a num- 

 ber of such occurrences in the Paleo- 

 zoic. There are the diffuse slightly curved 

 tubes of the worm Gitonia sipho which 

 traverse the fine-celled stratiform colonies 

 of the coral Stromatopora in the Silurian 

 and Devonian, the spiral worm Streptin- 

 dytes of graceful form in the Silurian 

 Stromatopora and one of heavier habit de- 

 scribed by Calvin in the large-celled coral 

 Acei'vularia of the middle Devonian. The 

 open apertures of tubes of Gitonia coral- 

 lopJiila are often to be seen projecting from 

 the cups and even the sides of the coral 

 Zaphrentis and its allies. These are simple 

 associations in which the partners are 

 and have always been equally and mutu- 

 ally dependent, both having begun life 

 as independent free swimming beings. 

 Among these commensal worms and corals 

 there is one association that invites par- 

 ticular notice in passing, for so far as I 

 know the present fauna, its parallel is not 



recorded. This is the compound coral 

 Pleurodictyum and its worm Hicetes — an 

 association long known to students of the 

 Devonian, but not till recently, with the 

 help of carefully prepared materials fully 

 comprehended. Pleurodictyum is a com- 

 pound favosite growing in small, large- 

 celled lens-shaped coralla of the size of a 

 brussels sprout. Its commonest species are 

 Pleurodictyum prohlematicum of the Euro- 

 pean Coblentzian and Pleurodictyum stylo- 

 ponim of the Hamilton shales of America. 

 The sessile thecated base of the corallum in 

 both these species seems to have attached 

 itself to the rock or mud of the sea bottom 

 in a way that involved the formation of no 

 special cicatrix or scar of attachment. My 

 observations lead me to the conclusion that 

 in the Coblentzian species such form of 

 attachment occurred in about one half of 

 the individuals and in perhaps somewhat 

 less than one half in the ITamilton species. 

 The rest attached themselves at the begin- 

 ning of the sessile stage to some organic 

 object, some dead shell on the sea bottom. 

 In the Hamilton species in approximately 

 one fifth of the cases, this dead shell was 

 usually a gastropod of the genus Pleuroto- 

 maria or Cyclonema; in the remaining four 

 fifths, the shell was always the same shell, 

 a species of the gastropod Loxonema 

 {delphicola = hamiltonensis) . I have not 

 recorded an instance of the coral being 

 attached to any other than a gastropod, 

 shell. These are rather broad statements 

 but are based on the examination of several 

 hundred examples and are an approximate 

 expression. These bases of attachment 

 were of course dead shells which the free- 

 swimming coral larvse selected. I have 

 italicized the word "selected" as, however 

 else one may be disposed to interpret the 

 phenomenon, the act bears a strange resem- 

 blance to conscious choice. The Cob- 

 lentzian Pleurodictyum so similar in struc- 



