170 THE STORY OF LIFE IN THE SEAS. 



it will be observed that this is in reality a cup- 

 shaped protrusion on the wall of the gill-chamber 

 covering a little, flat, soft animal. In past times it 

 was thought that this was a young flat fish, and 

 a wonderful story of its development was fabri- 

 cated on the strength of this error. It is now 

 known to be one of these extremely degenerate 

 Isopod parasites called Bopyrus. 



It is a curious fact that there is very rarely 

 indeed more than one of these parasites on a 

 single Prawn. If there is one in the right gili- 

 hamber there are none in the left, and vice versa. 

 It is difficult to find a satisfactory explanation 

 for this, for it is not at all probable that, during 

 the lives of the many hundreds of Prawns that 

 have been examined, only one larva has passed 

 through the gill-chamber of each individual. The 

 explanation must be looked for in some hitherto 

 unknown influence which the parasite has upon 

 the constitution of the host, rendering it unsuit- 

 able for the attachment of another Bopyrus of the 

 same habits. The case is by no means unique. 

 There are several instances of Fish and other 

 animals that bear one, and never more than one, 

 parasite of a particular species. 



A few words must now be added about the 

 internal parasites of marine animals. The sub- 

 ject is really an immensely wide one ; for the 

 intestines, body cavities, and even blood-vessels 

 of Fish are liable to the attacks of many different 

 forms of Flukes, Tape-worms, and other kinds of 

 parasites which are not even known by name, 

 perhaps fortunately, to the general public. 



The life-history of some Flukes that occur in 



