326 Mr. Jeffreys on Stilifer, 
truded for the purpose of seeking this nourishment. I neea 
not say that the reputation of Dr. Fischer as a physiologist, 
especially with regard to the Mollusca, makes any opinion of 
his on such subjects very valuable. I share his incredulity as 
to Stilfer bemg a parasite in the ordinary meaning of the 
word; but my impression is that it feeds on the excretions of 
Echinoderms, and not on animalcules or other organized and 
living matter with which sea-water abounds. It has never been 
found except on Echinoderms, or imbedded in their rays or 
spines. All the specimens of Stilifer Turtoni which I have seen 
in situ (and they have been rather numerous) occupied the upper 
sides of Echini, in the area of the vent or anal opening. The 
Echini so infested appeared to be invariably in perfect health 
and vigour. The Shetland specimen of EL. Drébachiensis was 
carefully watched by me for more than twelve hours. Its tubular 
suckers and pedicellarize continued in active although intermit- 
tent motion during all that period. The Stilifers were nestling 
or slowly crawling about among the spines; but they did not 
touch any of the suckers of the Echinus, which, being retractile, 
could easily have been withdrawn into the test; nor could I 
detect either of the mollusks in the act of feeding on the outer 
membrane or any other part of the Echinus. At the same time 
it is clear that there is some connexion between the peculiar 
habitat selected by the Stilifer and its food ; for if it subsisted on 
any living organisms, it would hardly confine itself to Echino- 
derms, but have a more varied range of habitat. Such shelter 
as an Echinus or Asterias could afford might be as easily obtained 
in crevices of rocks or in the cavities of deserted shells. Con- 
sequently, although I do not consider this a case of true para- 
sitism, like that of the mistletoe among plants, neither would I 
refer it to epiphytism, like that of a tropical orchid. It rather 
reminds one of the scavenger-habits of dung-beetles. 
‘I have elsewhere * endeavoured to show that the pretty little 
bivalve shell called Montacuta substriata, which also infests 
various Echinoids, is not really a parasite. This always occupies 
a different part of the Hchinus from that where the Stilifer takes _ 
up its abode; it adheres by its byssus to the ventral spines near 
the opening of the mouth on the under side. Here it probably 
avails itself of the current or indraught excited by the ciliary 
action of the Spatangus or other Echinoid for its own purposes ; 
and both partake of the same food in amicable but unconscious 
relationship to each other. As far as I have been able to observe, 
the Stilifer does not cause more inconvenience than the Monta- 
cuta to its not unwilling host. 
The suctorial proboscis, as well as the want of a denticulated 
* British Conchology, vol. ii. p. 208. 
