598 
SCANDINAVIAN FISHES. 
long has it been known that a singular and intimate 
relation exists between sea-cucumbers and certain fishes 
of this subfamily which in Cuvier" formed the sub- 
genus Fierasfer of the genus OpMdiiim. These fishes 
have the habit of seeking shelter and, to some extent, 
food inside sea-cucumbers and bivalves, where they lead 
a parasitic life, feeding upon the minute animals drawn by 
the current into the respiratory tree of the sea-cucumber 
or towards the mouth of the bivalve. Now and then 
they make sallies in search of food, but soon return 
to their hiding-place. 
whose habits are best known, the Mediterranean F. acus , 
usually makes its way in the same manner, according 
to Emery, tail foremost into the respiratory tree and 
body cavity of the Holothurice. Soft, almost transparent, 
and helpless, without weapons of defence, not even with 
scales to protect its body, and with powers of sight 
certainly feeble during the daytime, as shown by the 
contracted pupils, the fish feels its way, with head 
turned down, until with its snout and Avith the system 
of the lateral line it feels the current that passes into 
the hind part of a Holothuria. Noav the fish bends 
Fig. 
143. 
Fierasfer acus , in the act of entering a Holothuria. 2 / 3 of the natural size. After Emery. 
Hoav this habit has arisen, Ave can at least con- 
jecture from Verrill’s observation'' of a species of the 
preceding subfamily, Ophidium marginatum, in Great 
Egg Harbour on the coast of NeAv Jersey. “We dug 
tAvo specimens,” he Avrites, “out of the sand near Ioav- 
Avater mark, Avhere they burroAved to the depth of a 
foot or more. When placed upon moist sand they 
burroAved into it tail foremost Avith surprising rapidity, 
disappearing in an instant.” That species of Fierasfer 
the tip of its tail doAvmvards and inserts it into the 
cloaca of the sea-cucumber. In adult specimens of this 
species the tip of the tail is Avithout fin-rays, but the 
last rays of the dorsal and anal fins lie close to the 
body at this spot, so that the end of the tail forms a 
comparatively stiff point, Avhich can be inserted Avithout 
difficulty into the opening. By the breathing of the 
sea-cucumber and the undulating movements of the 
fish itself the tail is iioav forced farther and farther 
“ Regne Animal , ed. 2, tome II, p. 359. In Risso Fierasfer was the specific name of Fierasfer acus ( Ophiclium imberbe , Lin., p. p.). 
1 American Naturalist, vol. \ r (1871), p. 399. 
