64 
ORGANIC DEPENDENCE AND DISEASE 
EARLY PARASITISM OF THE SNAILS UPON THE 
CRINOIDS 
A true parasitic combination lias 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 which must have been 
obnoxious to it as it interfered with the normal alimentarv 
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 liolostomatous 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 pliylogeny and fossils show it to 
