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 Enlima ( 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. 
