FOURTH REPORT OF THE DIRECTOR I907 15 ^ 



viduals of this genus of shells. This combination makes its first 

 appearance in the early Devonic and seems to ha ve become intensified 

 in the great crinoid plantations of the early Carbonic but in either 

 formation the examples of the actual dependent combination are 

 in very slender proportion to the number of individuals of either 

 snail or crinoid. Some of the snails acquired this habit of parasitic 

 dependence, others evidently did not. Apparently it was in some 

 nieasure an individual adjustment. Yet the more general de- 

 pendence of this snail Platyceras on the crinoids is indicated by 

 the fact that quite generally Paleozoic strata carrying an abund- 

 ance of the one also abound in the other. 



Time has not extinguished this affiliation, for the existing seas 

 afford occasional evidence of similar relation between the limpets 

 and the crinoids. Our material seems to throw some light on the 

 inception of this dependent habit. A crinoid, Glyptocrinus, from 

 the Lower Siluric is occasionally found inclosing in its arms a 

 holostomatous snail, Cyclonema, not attached to the dome, for the 

 shell had not the limpet habit of attachment, but lying free in such 

 attitude as to get the full advantage of the crinoid's waste. 



True dependence is also indicated by a similar association be- 

 tween the crinoids of the- Carbonic rocks and the starfish Onychas- 

 ter. The starfish adjusts itself, mouth downward over the anal 

 aperture of the crinoid. Our specimens showing this condition 

 have been caught in this act of feeding. The flexible character 

 of the starfish made the attachment easily subject to change. This 

 association too is one that time has not cured. 



Much more abundant than these exhibitions of parasitism are 

 those of the commensal habit as indicated by our illustrations of 

 worms and corals, worms and sponges, barnacles and corals. 



In the natural and expected course of procedure commensalism 

 is the precursor of parasitism, and commensal associations became 

 established more abundantly and at an earlier date than the other. 

 Such mutual associations among members of the groups here indi- 

 cated have been continued till today, not in precisely similar mani- 

 festation but in like alliances between individuals of the different 

 divisions. 



The protected sedentary condition, effected either by the agency 

 of a special organ, as among most of the old brachiopods during a 

 part or all of their life, or by the cementation of the shell to the 

 rocks or some like object, is so widespread as to here command 

 attention as a still simpler expression of dependent life. That the 



