FOURTH REPORT OF THE DIRECTOR I907 I53 



faunas adapted themselves to life in shallow waters did the necessity 

 for shell protection arise for with this change from a free-swimming 

 to a creeping or stationary littoral habit came the lessened capacity 

 for escape by locomotion. As Lang has said, the coast is full of 

 dangers; the waves beat violently against it, the regularly re- 

 turning tide keeps the waters ever moving. From these attacks of 

 nature's blind forces the creatures must protect themselves. Some, 

 in times of stress, seek deep water, some scuttle into protected 

 spots or bury themselves in the sand, and others catch hold of 

 stable bodies, attajhing themselves by suction or fixation. But all 

 these resorts are inefficient without the addition of shell protection; 

 that once achieved, the animals may rejoice and flourish in the 

 play of the waves which brings them nourishment with decreased 

 exertion on their part. The primary step toward a degeneration 

 which in the lapse of ages has led to the dependent life conditions 

 of today would seem with reason to lie in the forced reduction of 

 this locomotive power and adaptation to a sedentary condition 

 resulting in the necessity for the formation of a protective shell. 



ILLUSTRATIONS OF PALEOZOIC SYMBIOTIC ASSOCIA- 

 TIONS 



The instances here given are some of the more instructive occur- 

 rences of this sort that have come under my notice. They are not 

 in all cases common though they exemplify consociations which are 

 familiar in like groups of the living world. The record of their 

 number will doubtless be much increased as such objects come under 

 closer observation. The collection of such data from the early 

 periods of the world's life is not likely to be carried too far for it is 

 here, rather than in a profuser and much more complicated later 

 development, that the factors of symbiosis are the more easily 

 legible. 



Worms and Corals 



The coexistence of the tubicolous worms with the corals is one of 

 the commonest phenomena of present seas. It became established 

 at a very early stage in the earth's history and in the Devonic coral 

 reefs the habitude had already become widespread and varied. It 

 was palpably less frequent in Siluric times, at least our material 

 would so indicate, and when it does present itself, the expression 

 is quite simple. In most cases it is an elementary expression of 

 commcnsalism. Worm and coral may start to grow together di- 



