MUTUALISTS. 75 



Naturalists had recognized some crustaceans under 

 the name of Ancei, and others under the name of 

 Pranizse, living together upon fishes, but with very- 

 different organs for fishing and swimming. M. Hesse, 

 curious to know the manner of life of the Pranizae, made 

 observations on them in a small aquarium, and he per- 

 ceived that the parts of the mouth were all at once 

 transformed into formidable mandibles, which caused 

 them to resemble Ancei. As it had often occurred with 

 respect to other groups, that the same crustacean at 

 different periods of its evolution had been taken for 

 different animals, the naturalist of Brest had some sus- 

 picion as to their identity, and soon ascertained by direct 

 observation that he had not been mistaken. The Pranizse 

 become Ancei, and live upon fishes under their first 

 form, like caligi and arguli. Nothing can be seen 

 which is more curious than these crustaceans, which 

 ride on the back or the sides of fishes, and assume there 

 every possible attitude. 



The Pranizse fix themselves in the mouth and in the" 

 gills as well as on the skin. Some are found on 

 sharks as well as on osseous fishes. They fear neither 

 heat nor light, and do very well under damp sea-weed 

 while waiting for the return of the tide. They run and 

 swim with the same facility. When in the condition of 

 Ancei, they lose their agility, and, under this form, all 

 denotes their sedentary habits. They appear to live in 

 holes, at the bottom of which they defend themselves 

 with their powerful mandibles. It has been observed 

 that fecundation is accomplished, as in the Axolotls, 

 before the evolution is- complete, but that the eggs are 

 not laid until the animal assumes the form of Anceus. 



