MUTUALISTS. 81 



Its rank and its affinities would have given rise to 

 long discussions if we had not made known at the same 

 time its evolution and anatomical structure. 



It is neither a parasite nor a messmate ; it does not 

 live at the expense of the lobster, but on one of the pro- 

 ductions of these crustaceans, much in the same manner 

 as do the Caligi and the Arguli. The lobster gives him 

 a berth, and the passenger feeds himself at the expense 

 of the cargo ; that is to say, he eats the eggs and the 

 embryos which die, and the decomposition of which 

 might be fatal to his host and his progeny. These 

 HistriobdellsB have the same duty to perform as vultures 

 and jackals, which clear the plains of carcases. That 

 which causes us to suppose that such is their appro- 

 priate office, is that they have an apparatus for the 

 purpose of sucking eggs, and that we have not found in 

 their digestive canal any remains which resemble any 

 true organism. We find the feces, rolled up as balls, 

 placed after each other in their intestines. 



The crustaceans also feed other Hirudinidas. Mons. 

 Leydig has noticed a Myzpbdella on the Lwpa diacantha. 

 The fresh-water crab, common in all the rivers o^ 

 Europe, nourishes two, the Astacobdella rceselii, which 

 lives under the abdomen, or about the eyes, and the 

 Astacobdella Abildgardi which especially frequents the 

 branchiae. Two astacobdella on the same crab doubt- 

 less play different parts. We should almost venture to 

 assert, a, priori, that the species in the gills lives as a 

 parasite on the blood of its host, whilst the other, lodged 

 under the abdomen, plays the same part as the histrio- 

 bdella of the lobster. 



We often find among the eggs of the ordinary crab of 



