13 



is at tlie apex of a small pajjilla, and (•omiiiiiuicates with 

 a sac in the interioi (Plate II., tig. 11). 



When resting, Lepeoyhthvivu.'^ lies upon the vential 

 surface, keeping- the first three pairs of feet moving- with 

 spasmodic jerks. When irritated, as in attempts to remove 

 them from their host, the males and immature females 

 move very rapidly over the skin of the fish. The mature 

 females make no attempt to escape, only clinging- more 

 securely. On transferring them to clean sea water they 

 settle on the sides and bottom of the vessel, and sometimes 

 adhere to the surface him of the water, remaining quiescent 

 for long periods. When the water is shaken slighth' they 

 detach themselves and swim about rapidly on their backs. 

 They soon tire, however, and return to rest again. Lepeo- 

 phtheirus makes no attempt to leave the water when kept 

 in small aquaria. The allied form, C ally us, on the other 

 hand, crawls out of the water and up the sides of the 

 glass, where it remains, making no attempt to return, and 

 soon dies owing to the evaporation of the water from 

 under the carapace. These parasites are very tenacious 

 of life, and live for a considerable time after the host has 

 died if they are not allowed to dry up. In some instances, 

 although the host had been dead over twelve hours, and 

 the parasites to all appearance were also dead, they 

 soon revived when placed in sea water. Increase of 

 temperature to 1()° ('. and over is fatal to them. They 

 can, hoAvever, stand very considerable decrease of tem- 

 perature. On one or two occasions during February, 

 1900, the small aquaria in the tank room at Piel, some 

 of which contained parasites under observation, were 

 frozen, and the temperature of the room itself stood at 

 — 1° C, but the parasites suffered no harm. They can also 

 be kept alive in sea Avater for weeks without change if the 

 aquaria are kept cool. 



