SEA-FISHERIES LABORATORY. 191 



attacked to it, each pair indicating a somite. At one end 

 of the series, these parasites approach very nearly in 

 structure and general appearance to the non-parasitic 

 Copepods. At the other end they are extremely different, 

 exhibiting most remarkable examples of retrograde 

 development, and without a complete study of their life 

 history it would be quite impossible to recognise them 

 even as Crustacea. 



In the following pages an account is given of the 

 anatomy and metamorphosis of one member from each of 

 the two very different families, the Caligidae and the 

 Lernseidse, the forms chosen being Lepeophtheirus 

 pectoralis and Lerncea branchialis. 



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 Caligidae 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 Caligus. There are, 

 however, 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 separate genus. Caligus has two semicircular 

 suckers on the frontal margin of the cephalic shield, which 

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

 and the biting part of the second maxillae has only one 

 tooth. In Lepeophtheirus these suckers are entirely absent 

 all 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 first becomes attached to its host. 

 (see p. 219). 



