304 FISHES 



respiratory trees. More than one fish may enter successively 

 into the same Holothurian ; in the aquarium at Naples, Prof. 

 Emery saw as many as seven pass into the same animal one 

 after the other ; but in Holothurians captured in the sea, never 

 more than three were found together, and when larger numbers 

 entered the host soon died. The fish are only found in Holo- 

 thurians captured in deep water, those found near shore being 

 always free from them. The entrance of the fish is facilitated 

 by the respiratory movements of the host, which is obliged at 

 intervals to dilate its anal aperture for the expiration and in- 

 spiration of water. The fish obtains no food or nourishment 

 from its host, its stomach being always found to contain crus- 

 taceans which are not to be obtained from the intestinal cavity 

 of the Holothurian. It seems probable that the fish is in the 

 habit of leaving its host to catch its prey, and that it does this at 

 night, retiring into its living shelter at the approach of daylight, 

 but this has not been observed ; all that has been seen is that 

 the Fierasfer sometimes protrudes its head and front part of its 

 body from the anal aperture of the Holothurian ; it could 

 scarcely catch Crustacea in such a position. In Holothurians 

 captured from the sea the Fierasfer is usually found in the body 

 cavity, into which it can only pass by rupture of the delicate 

 walls of the respiratory tree, but this apparently causes no great 

 harm to the Holothurian. The fish is not, properly speaking, 

 a parasite, since it does not feed on its host, and it is not what 

 is called a " commensal " or messmate, because it appears to 

 confer no benefit on its host, it is merely, as Prof. Emery says, 

 an uninvited guest. Its existence within the Holothurian evi- 

 dently depends on the peculiar mode of respiration of the latter, 

 the fish obtaining its oxygen from the water inspired by its 

 host. The fish might thus be called a respiratory parasite, but 

 the origin of the habit is merely to be attributed to the instinct 

 of the fish to seek shelter from its enemies in holes and crevices, 

 as do the conger and Murcsna and many other fishes. 



Off the coasts of Japan a species of Fierasfer lives not only 

 in Holothurians but also in the body of globose starfishes of 

 the genus Culcita, and on the tropical coast of America Fierasfer 

 dubius is found within the valves of the pearl oyster, Melea- 

 grina margaritifera, and occasionally the dead body of the fish 

 has been found in the shell entirely enclosed in mother of 



