COMMENSALISM AND PARASITISM. 167 



only in sea- water. It is difficult to give an exact 

 statement as to its size, because, like all its re- 

 lations, it is capable of very extensive movements 

 of expansion and contraction, but the Pontobdella 

 is large for a Leech, and when moderately con- 

 tracted it may be two or three inches in length. 

 The body of this Leech is covered with small 

 tubercles, and it has a large round sucker at 

 each end. Its favourite hosts are the Sharks 

 and Kays, but as it usually drops back into the 

 water when these Fish are hoisted on to the deck 

 it is not very commonly seen in the fishermen's 

 boats. 



The most common external parasites of Fish 

 are the Fish-lice. Most of these are little Crus- 

 taceans, belonging to a group which includes the 

 Wood-louse. They have curiously flattened 

 bodies, provided with short, bent legs, termin- 

 ating in sharp hooks, by which they adhere to 

 the body of the Fish and crawl about over the 

 skin. Some of these parasites seem to prefer the 

 tongue as a resting-place, the genus Glossobius, 

 for example, being found in this position on the 

 Flying-fish of both the Pacific and Atlantic Oceans. 

 In Glossobius we find a very remarkable difference 

 in size and form between the males and females, 

 a condition of affairs which is of very common 

 occurrence among the parasitic Crustaceans. The 

 male in this particular case is so small that it is 

 entirely concealed beneath the tail of the female. 

 In another genus a still more interesting con- 

 dition has been observed, the small young forms 

 which are males growing up, and changing in 

 later life into females. 



