74 ANIMAL PAKASITES AND MESSMATES. 



sticklebacks, and on the greater part of our river, fish. 

 Mr. Thorell, in his monograph, mentions twelve species 

 of Arguli proper, and four species of which he com- 

 posed the genus Gyropeltis. Pour are found in Europe, two 

 of which are on salt-water, and two on fresh-water fish. 



Quite recently, Professor Leydig has made known 

 another species living on the Phoxinus levis. Arguli are 

 met with on the fishes of Africa, the Indies, and North 

 and South America. Like the caligi, these animals 

 spontaneously abandon one host, to go and attend to 

 the toilet of another. 



Another animal, which has been taken for a Lernaean, 

 deserves to take its place by the side of the Caligi, at 

 least on account of its manner of life. We refer to that 

 singular being which Leydig discovered in 1850 in Italy, 

 while studying the mucous canal of a Corvina, at 

 Cagliari, and to which he gave the name of Sphcerosoma. 

 To judge by the plate and by some details, this 

 Sphcerosoma, the name of which ought to be changed to 

 Leydigia, belongs, if we mistake not, to the same group 

 as the Histriobdellas. We are persuaded that the first 

 opportunity will confirm the correctness of this alliance, 

 by the study of its embryonic form. If we had not been 

 able to examine into all the development of the Histrio- 

 bdellee, more than one naturalist would have considered 

 them Lernsans, as happened at the congress of German 

 naturalists at Carlsruhe. 



If we see many of these crustaceans live a joyous life 

 while young, there are others which seem to practise 

 economy, and to emancipate themselves when they have 

 grown old. Mons. Hesse and Mr. Spence Bate a few 

 years since revealed the secrets of their existence. 



