PARASITES DURING THEIR WHOLE LIFE. 259 



We may place at the head of this group the Tristomum, 

 ■which has only been discovered a few years. We owe 

 to Baster the knowledge of a beautiful and large species, 

 which inhabits the body of the halibut. Naturalists 

 have given it the name of Epibdella, This worm is of 

 the size of the human nail ; it resembles in form a box 

 leaf; by the aid of its suckers it clings to the skin of its 

 host like a scale ; and is sometimes mistaken for one. 

 It is of an oval form, and of a dull white colour ; it can 

 scarcely be distinguished from the skin of the fish. "We 

 may have it before our eyes for a long time before we 

 perceive it. 



Another Epibdella lives on the skin and on different 

 parts of the body of the European maigre, or the Virgin 

 Mary's fish ; it is covered with pigment spots which 

 cause it still more to resemble the large scales of its host. 

 This fish, which is also called the Scisena aquila, has its 

 skin covered with similar scales, and they are of the 

 same colour, both on the back and belly. 



Another large and fine worm of this group lives on 

 the gills of the sturgeon, and is distinguished by its 

 suckers as well as by its great mobility. The epibdells 

 preserve their scale-like form during their greatest con- 

 tractions, but these worms change with every movement. 

 The Nitschia elegans, for such is the name by which it 

 is distinguished, is not rare on the sturgeon as we see 

 it in our markets. Among the many parasites in this 

 category, there is a very remarkable one which deserves 

 particular mention. It lives abundantly on fresh-water 

 fishes, preferring to attach itself to their gills ; it is found 

 most commonly on the bream. For our knowledge of 

 these worms we are indebted to Nordmann. 



