attached to it, eaoli ]iair indicating a somite. At one end 

 of the series, these ])arasites approach very nearly in 

 strnctnre and g'eneral appearance to the non -parasitic 

 Copepods. At the other end they are extremely different, 

 exhibiting' most remarkahle examples of retrograde 

 deA-elopment, and withont a complete stndy of their life 

 history it would be (juite impossible to recognise them 

 even as Crustacea. 



In the following pages an account is given of the 

 an atom \- and metamorphosis of one member from each of 

 the two very different families, the Caligidse and the 

 Lernseidse, the forms chosen being Le-peophtheirus 

 pectoralis and Lcriued hv<(in-]ii(dis. 



The Caligidae is the most extensive family of the Cope- 

 pod fish parasites, and contains a larger number of genera 

 and species than any of the others. As it stands at 

 present, there are 124 species representing 25 genera. 

 Three-fifths of the known species of CaligidtB belong to 

 two genera, Caligus and Lepeophtheirus. Some earlier 

 authors have not recognised the latter genus, and include 

 the various species belonging to it in Cdligus.. There are, 

 hoAvever, very important differences between the two 

 which make their appearance early in life. These 

 differences are constant, and give good cause for establish- 

 ing a separ?<le genus. Ca/if/its has two semicircular 

 suckers on the frontal margin of the cephalic shield, which 

 are developed before the " chalimus "* stage is completed, 

 and the biting part of the second maxilhe has only one 

 tooth. In Lepeop/ifheirus these suckers are entirely absent 

 ail through life, and the biting part of the second maxillae 

 has two teeth. The changes that take place between the 

 " nauplius " stage, when the animal is hatched from the 



* The stage at which the animal hist becomes attached to its host. 

 (see p. 32). 



