64 ORGANIC DEPENDENCE AND DISEASE 



EAELY PARASITISM OF THE SNAILS UPON THE 



CRINOIDS 



A true parasitic combination has long been recognized 

 among the ancient crinoids or sea lilies and the gastropods 

 or sea snails. It began, so far as we know, in the Silurian, 

 became of common occurrence in the Devonian and reached 

 a climax in the Carboniferous age ; and with the close of 

 Paleozoic time, this special combination ceased or was ex- 

 tinguished. The history of these cases is of very large sig- 

 nificance to our investigations. The crinoid, by the end of 

 the Ordovician period, was fully developed and specialized. 

 Its habit, presumably from the larval state on, as today, was 

 one of fixation by a long highly flexible stem which re- 

 mained functional, with rare exceptions if at all at this 

 period, throughout the rest of life. It had developed a well- 

 defined aboral or anal orifice to the curved intestine and 

 this opening was on the upper surface of the calyx between 

 two of the arms or in the summit of the dome embraced by 

 these free arms. In many species of the later Carbonifer- 

 ous and Devonian this dome was elongated into a proboscis 

 which carried the anal aperture upon its summit either for 

 the purpose of conveying the rejectamenta beyond the 

 reach of the mouth or for the actual protection of the crin- 

 oid host from the parasitic gastropod whiqh must have been 

 obnoxious to it as it interfered with the normal alimentary 

 function. These suggestions will be again referred to. 



The gastropod had also attained in the Ordovician a nor- 

 mal form, that of a simple spiral shell with a circular 

 aperture or mouth. It was a holostomatous shell, with a 

 continuous shell mouth, without any special adaptation for 

 fixation by suction or otherwise to any extraneous object. 

 The limpets had already developed, and these are shells 

 which express even better the simple conical form out of 

 which all the coiled gastropods had been derived. This type 

 was primitive and both phylogeny and fossils show it to 



