March, 1898.] 61 



NOTE ON A POST-I.ARVAI. FIERASFKR. 



BY PROFESSOR W. C M'INTOSH, M.D., I.I..D., F.R.S. 



[Plate 2]. 



Amongst the fishes no more curious forms occur than those 

 which are commensalistic in the bodies of other animals. 

 They are found, Dr. Giinther tells us, in the Mediterranean, 

 Atlantic, and Indo-Pacific seas. Foremost in interest is the 

 genus Fierasfer, the Mediterranean species of which form the 

 subject of one of the beautiful monographs (by Prof Carlo 

 Emery) issued by the Naples Zoological Station.^ In this 

 work the author mentions F. acus as frequenting Holothiiria 

 ttibulosa and Stichopus regalis in the Mediterranean. The 

 former species also gives shelter to F. dentahis. Other species 

 have elsewhere been found in Meleagrhia maigaritifera, and it 

 has happened that a dead example has, like the Chinese 

 josses, been fixed to a valve by the pearly secretion, and also 

 in Asterias giobosa and Culcita, in which Doleschall says the 

 fish betakes itself to the stomach. In holothurians the fishes 

 appear to frequent the cloaca and respiratory passages, prob- 

 ably giving their host no more inconvenience than the 

 Pea-crab does the Mussel, or Nereilepas the Hermit-crab. 

 Emery observes that it may seek such a position for shelter 

 from predatory fishes, just as the earthworm takes to the 

 ground, or, as we should say, just as Amphioxus and the 

 Sand-eel take to the sand. The genus Fierasfer, however, is 

 less modified in external appearance than Encheliophis, an- 

 other commensalistic form in which the pectorals as well as 

 the ventrals have disappeared. 



Fierasfer seems to introduce itself — tail first — into the cloaca 

 of the holothurian at the moment of the issue of the excurrent 

 stream of water, and keeps its head to the incurrent stream 

 when in position — for respiration and it may be for food. 

 The latter consists of crustaceans such as schizopods, which 

 it appears to procure in a free condition. 



Prof Emery considers that a mass of floating ova in mucus 

 83 mm. long by about 41 mm. in diameter belongs io Fierasfer 

 acus. Towards maturity the eggs show groups of dull 

 yellowish oil-globules, which, as in the Sand-eel and other 



J , - - - - - III I 1 - ~ — 



■< Le specie del genre Fierasfer del Golfo di Xapoli ; I^eipzig, 1S80. 



