598 



.SCANDINAVIAN FISIIKS. 



lung li;is it l)ceii kinnvii tliat a singular and intimate 

 relation exists between sea-cucumbers and certain fishes 

 of this suljfaniily which in Cuvieh" formed the sub- 

 genus Fierasfer of the genus OpJiidiioii. Tliese iishes 

 ha\(' the habit of seeking shelter and, to some extent, 

 food inside sea-cucumbers and bivalves, where they lead 

 a parasitic life, feeding u])on the minute animals drawn bv 

 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, l)ut soon return 

 to their hiding-place. 



whose habits are best known, tlic Jlediteri-anean F. acns, 

 usually makes its way in the same manner, according 

 to Mmkuv, tail foremost into tlie resj)iratoi-y tree and 

 body cavit}- of tlie Holotlmria'. Soft, almost transparent, 

 and helpless, without weapons of defence, not even with 

 scales to ])rotect its l)ody, and with ])owers of sight 

 certainly feeble dui-ing the daytime, as shown ]t\ the 

 contracted pupils, tlie iisii feels its \\a\-, witii head 

 turned down, until with its snout and with the svstein 

 of the lateral line it feels the current that passes into 

 the hind part of a Holothuria. Now the fish l)ends 



Fig. 14.3. Fieraffci' aciis, in the .net of entering a Htilotliuria. 



of the nalnral size. After Ejiliny. 



How this habit has arisen, we can at least con- 

 jecture from \'ei!1^ill'.s observation'' of a species of the 

 preceding subfamily, Ophidium tnarginatum, in Great 

 Egg Harbour on the coast of New Jersey. "We dug 

 two specimens," he writes, "out of the sand near low- 

 water mark, -where they burl•o^ved to the depth of a 

 foot or more. When placed upon moist sand they 

 burrowed into it tail foremost Avith surprising rapidity, 

 disappearing in an instant." That s})ecies of Fierasfer 



the tip of its tail downwards and inserts it into the 

 cloaca of the sea-cucumber. In adult specimens of this 

 species the tip of the tail is -without fin-rays, but the 

 last rays of tlie dorsal and anal fins lie close to the 

 Ijody at this s])ot, so that the end of the tail forms a 

 comparatively stiff point, which can be inserted without 

 difficulty into the opening. By the breathing of the 

 sea-cucumber and the undulating movements of the 

 fish itself the tail is iww forced farther and farther 



" Rigne Animal, ed. 2, tome II, p. 3.5'.). In Ri.sso Pieras/er was the specific name of Fierasfer aais {Opliidiiiin imherbe. Lin., p. p.). 

 '' American Naturalist, vol. V (1871), p. 399. 



