66 
attached 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 Caligide and the 
Lerneide, the forms chosen being Lepeophthewrus 
pectoralis and Lernea branchialis. 
The Caligide 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 Caligide 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 “ chalimus’’* stage is completed, 
and the biting part of the second maxille has only one 
tooth. In Lepeophtheirus these suckers are entirely absent 
all through life, and the biting part of the second maxille 
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. 94). 
