ORGANIC DEPENDENCE AND DISEASE 65 



have 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^) 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 Echinoderma (starfishes, sea 

 urchins, etc.), the group to which the crinoids belong. Ton- 

 niges has given^ 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 N aturwissensch. Wochenschr., v. 19, no. 16, January, 1904. 



