ORGANIC DEPENDENCE AND DISEASE 
65 
liave been the most ancient. The first simple coiling of the 
shell which resulted from subsequent obstructed growth on 
one side of the animal could not have much altered any 
functions. The habit of these limpetlike shells with apical 
coil only (Platyceras, Capulus 1 ) seems to have been then, 
as now, one of easy but not necessary muscular attachment 
to whatever body they might choose to grow on, and today 
we have a long list of genera, both of marine and fresh wa- 
ters, in which this exterior form of shell and the internal 
habit are retained. 
In the various expressions of conjugation between such 
shells and the Crinoidea we have evidence enough to trace, 
step by step, the progress of free and haphazard associa- 
tion into a fixed and dependent parasitism; a cinema of 
some millions of years, displaying the gradual yielding to a 
fixed and essentially vicious habit. It is the illustration, 
critical we think, as it could hardly be derived except from 
the paleontological record, of the passage of symbiosis into 
parasitism. With it the case would seem to have been made 
out also for the incurability, under unconscious agency, of 
such a “perturbation of normal activities ” ; for so far as 
we can see the habit ended only with the downfall of those 
crinoidal groups whose structure exposed them to these 
attacks. 
We may here turn aside to note briefly the fact of para- 
sitic attachments in the existing fauna, of gastropods to va- 
rious representatives of the Ecliinoderma (starfishes, sea 
urchins, etc.), the group to which the crinoids belong. Ton- 
niges has given 2 a very interesting pictorial account of 
these, drawing in some measure on the determinations 
made by the Sarasin brothers. We are not much in the way 
of thinking of the snails as true parasites, but the author 
cited remarks that all of the very few species known to have 
1 True capulids of this period are doubtless reversionary forms as shown by 
tendency of later growth to reassume the conical shape. 
2 Natunvissensch. Wochenschr., v. 19, no. 16, January, 1904. 
