INTRODUCTION. 7 



and though adult and immature specimens of both 

 sexes may be found scattered all over the body, mature 

 egg-bearing females are most frequently met with 

 under the pectoral fins. (See Plate A.) It is possible 

 to collect a fairly complete series of stages of develop- 

 ment from the one host. The average length of a 

 mature egg-bearing female is one-fifth of an inch, and 

 of a male one-ninth of an inch. The parasites attach 

 themselves to the fish by means of their powerful 

 second maxillipeds assisted by the antennae, and a 

 decided pull has to be applied before they can be 

 removed. By depressing the edge of the carapace 

 and applying it closely to the skin, the parasite is able 

 to increase its holding power to such an extent that 

 the posterior portion can be torn off from the anterior 

 without detaching it. The anterior portion when thus 

 separated will sometimes remain alive for twenty-four 

 hours. It is even capable of swimming about 

 vigorously, but eventually its movements become 

 erratic and finally it dies. Lepeophtheirus pectoralis 

 can be kept alive in sea water for about six weeks after 

 removal from the host. The parasites live for a con- 

 siderable time after the fish has been caught and re- 

 moved from the water, if they are not allowed to dry up. 

 Increase of temperature to 16 C. is fatal to them. 

 Decrease of temperature to even below freezing-point 

 does not appear to incommode them. If the aquaria 

 are kept cool, the sea- water in which the parasites 

 are placed after removal from the host need only be 

 changed at long intervals. 



EXTERNAL APPEARANCE AND STRUCTURE. 

 The animal is depressed dorso-ventrally and is 

 divided into four parts. The first and usually the 



