Among the vertebrates, there are few except fishes which merit a place here; it is only amongst these that we meet with species at the mercy of others, and dependent on acolytes, which are in every respect inferior to themselves. An interesting messmate belonging to this first category is a fish of graceful form, named donzelina, which goes to seek its fortune in the body of a holothuria. Naturalists have long known it under the name of Fierasfer. It has a long body like that of an eel, entirely covered with small scales; and as it is quite compressed, it has been compared to the sword which conjurors thrust into their oesophagus. They are found in different seas, and all have similar habits. This fish is lodged in the digestive tube of his companion, and, without any 5 regard for the hospitality which he receives, he seizes on his portion of all that enters. The Fierasfer contrives to cause himself to be served by a neighbour better provided than himself with the means of fishing. Dr. Greef, at present Professor at Marbourg, found at Madeira a holothuria of a foot in length, in which a vigorous Fierasfer lived in peace. Quoy and Gaimard, in the account of their voyage round the world, have remarked long since, that the _Fierasfer hornei_ is found in the _Stichopus tuberculosus_. The holothuriæ seem to exist under very advantageous conditions in this respect, since we see Fierasfers, which are themselves tolerable gluttons, accompanied by Palæmons and Pinnotheres in the same animal. Professor C. Semper has seen holothuriæ in the Philippine Islands which bore a considerable resemblance, in this respect, to an hotel with its table d'hôte. These singular fishes have been long noticed, but it was not till recently that their presence in a host so low in the scale as a holothurian could be explained.