14 



adult is represented by a very similar structure with five joints, having 

 two terminal hooks and a taste-pad. 



On the third day an important change takes place, when the argulus 

 makes its first moult. It then measures i mm. The first and second 

 pairs of setee are shed, the second pair of clawed antennae still re- 

 maining. The four pairs of swimming-feet proper are now developed, 

 the basal joints carrying two others, each of which has hairs, one 

 singly, the other divided into two branches, which in their turn are 

 covered with minute secondary hairs, which require the higher powers 

 of the microscope to resolve. The first pair of swimming-feet at this 

 stage are more developed than the others, and up to the eighth day 

 are the only pair in active motion. At the apices of the lobes of the 

 tail-plate are two tubercles, each terminated by three hairs ; at later 

 stages these gradually recede between the lobes. Their use I am 

 unable to determine. The first pair of foot-jaws have now four barbs 

 to each claw, as well as an adjacent plain one. Surrounding the 

 mouth, and attached to the base of the foot-jaws, are several chitinous 

 teeth. These are probably used for making the incision in the body 

 of the host prior to feeding. 



The second moult corresponds to the third stage in the life of the 

 larva, and occurs from the eighth to the tenth day after hatching 

 (Fig. 4). The increase in size is but slight, being only i '5 mm. The sex 

 may now be determined by the presence of the black spots on the 

 tail-plate in the case of the female. The male organs are not yet 

 developed. The most notable change is at the base of the first pair 

 of foot-jaws, which are now becoming enlarged preparatory to a 

 remarkable change which occurs after the next moult. On the 

 under surface of the front of the carapace, a few spines appear, 

 whilst the swimming-feet are gradually developing hairs and per- 

 forming their proper functions. The second pair of antennal hooks 

 are still retained, and the tubercles at the extremities of the tail-plate 

 have approached nearer the fork of the two lobes. 



The third moult, or fourth stage, commences about the sixteenth 

 day, and a considerable increase in size is noticed (2 "5 mm.). The 

 male organs are also evident. The second pair of antennal hooks 

 are cast, as are also the first pair of foot-jaws. These have become 

 modified into sucking-discs, which have been gradually developing 

 for the past three or four days at the base of the foot-jaws. Portions 

 of the hooks still remain attached to the margin of the discs. 



About the twenty-fourth day the fifth stage begins. The most 

 important change is the shedding of the hooks above mentioned, 

 the merest rudiments remaining. The first and second pairs of 

 swimming-feet have each developed a small, recurved branch, which 

 arises from the base of the fork of the two main branches on the 

 dorsal side. The growth is now very rapid. 



A few days later a fifth moult takes place. The suckers have now 

 thrown off the rudiments of the hooks, and the Argulus has reached 

 the perfect state. Its measurement at this stage is from 4 to 5 mm. 



