66 ORGANIC DEPENDENCE AND DISEASE 



parasitic habits live on the echinoderms — as though the 

 habit established in ancient Paleozoic days upon the crin- 

 oids had found favorable adjustment throughout the group. 

 Nevertheless it is of special interest to find that one of these 

 parasites is a limpet of essentially the same type as those 

 which infested the ancient crinoids. This is the species 

 Thyca ectoconcha which attaches itself to the outer skin of 

 a starfish and inserts an extension from its snout well into 

 the integument. The interesting statement is made that 

 Thyca is usually a wandering snail crawling over the spines 

 of the starfish, devouring the little sponges and their ad- 

 herent organisms ; in other words, lives normally until it 

 adopts the attached and parasitic habit. The starfish has a 

 still more degenerate parasite-snail, Stilifer, of a distinct 

 generic type with spiral coil, which sinks itself deep in the 

 skin of its host leaving only a minute projection of its spire, 

 while the rest of the shell becomes enveloped in a false man- 

 tle or membrane from which a proboscis develops that pene- 

 trates far into the soft parts of the host and sucks out its 

 nourishment. Far more extreme cases of degeneration are 

 illustrated by the snails Entocolax and Enteroxenos, para- 

 sitic within the sea cucumbers or holothurians and which 

 appear to have found their way in by penetration through 

 the wall of the body. In these cases the gastropod struc- 

 ture is so modified in the adult, by loss of form, shell, or 

 dental ribbon and indeed the adaptation of all the anatomy 

 to a suctorial function, that identity is shown only by the 

 normal stages through which the young shells pass. 



Living crinoids do not escape similar infestation but not 

 by the limpetlike snails. A. H. Clark has found Ptilocrinus 

 pinnatus parasitized by the spiral Eulima (ptilocrinicola) 

 which Bartsch, who describes the species, finds attached to 

 the calyx through a veritable puncture of the skin made by 

 a suctorial proboscis. These extreme adaptations indicate 

 complicated conditions of parasitism which could not well 

 belong to the less complex life of the Paleozoic, 



