32 ANIMAL PARASITES AND MESSMATES. 



the Boops, and on the Trachina vipera. These cymothoes 

 are about fifteen millimetres in length, and often fill all 

 the cavity of the mouth. The most curious of all is 

 that which is found in the mouth of the flying-fish, a kind 

 of herring with elongated fins, which it uses as wings to 

 rise into the air, when too closely pursued in the water. 

 My son, when examining these fishes, in his passage 

 from Cape Yerd to Eio de Janeiro, found in the cavity of 

 their mouth an enormous female, firmly wedged in the 

 branchial arches, with its head inclined outwards, and 

 the male, which was rather smaller, installed at her side. 

 Their dwelling thus by pairs, as well as the entire con- 

 formation of the animal, plainly shows that these crusta- 

 ceans make themselves at home, and live as true mess- 

 mates. Cunningham has given them the name of 

 Ceratothoa exoceti. A short time since, these Cymothoes 

 were only known on marine fishes, but it appears from 

 recent observations, that fresh-water fish are far from 

 being exempt from them. Mons. Gertsfeld has recently 

 noticed some on the Cyprinus lacustris of the river 

 Amour, and another in the Eio Cadea in Brazil, on a 

 Chromida. Other isopods also resort to fishes, and to 

 animals of their own class, but they live as true para- 

 sites, and change their form as soon as they have 

 chosen a resting-place. We shall return to this subject 

 again. Some which are very common on prawns, are 

 known under the name of Bopyrus. 



An interesting division of amphipods have received 

 the name of Hyperinm. These crustaceans generally 

 swim with facility, but walk with difficulty. They there- 

 fore usually have recourse to fishes, or even to medusas, 

 in order to gain support. We find on our own coasts the 



