INTRODUCTION. 9 



posterior margins. When the spot is examined micro- 

 scopically it is found to consist of two distinct eyes 

 closely approximated and embedded in a mass of 

 reddish-black pigment. The eyes are wholly under 

 the carapace. Each eye has a simple, spherical, crys- 

 talline lens beneath a thin cornea. Behind the lens 

 lies a row of retinal cells of fairly large size and lined 

 internally with a layer of pigment. A chitin division 

 deeply pigmented with red separates the two eyes. 



The Appendages. 



There are twelve pairs of appendages, as follows : — 

 One pair of antennules, one pair of antennas, one pair 

 of mandibles, two pairs of maxillse, two pairs of maxil- 

 lipeds, and five pairs of feet. The first three pairs 

 of feet only are adapted for swimming. The fifth pair 

 of feet is very small and is attached to the postero- 

 lateral margins of the genital segment. A sixth pair 

 of feet is usually present in the male, appearing as 

 small papillae immediately behind the fifth pair. 



The antennules are placed on the frontal margin of 

 the cephalo-thorax and are two-jointed. The basal 

 joint is usually moderately broad and is furnished with 

 a number of plumose setae. The setas are well supplied 

 Avith nerves and evidently act as sensory organs. The 

 other appendages are all on the ventral surface. The 

 first in order are the antennas. These are two-jointed. 

 The apical joint is a strongly-prehensile claw which is 

 used for holding on to the host. The mandibles are 

 stylet-shaped and are enclosed in the suctorial mouth. 

 The apical joint is flattened and curved. The inner 

 margin is distinctly serrate at the distal end, the 

 serrations being sometimes more or less irregular. 



The mandibles of all the copepod parasites of fishes 



