8 BRITISH PARASITIC COPEPODA. 



largest is almost circular in outline. It bears all the 

 appendages with the exception of the fourth and fifth 

 pairs of feet. This part is known as the cephalo-thorax. 

 The second part is very small and represents the fourth 

 thoracic segment of the pelagic Copepoda. It bears 

 the fourth thoracic feet. The third part of the body 

 is of variable size and shape, and is known as the 

 genital segment. Its size and shape depend largely 

 upon the degree of maturity of the reproductive organs. 

 The fourth part is short and narrow. It is only about 

 one-fourth the width of the genital segment. It repre- 

 sents the abdomen of the pelagic Copepoda, and bears 

 at its apex two short papillee known as the f ureal joints 

 or caudal stylets. 



Viewed from above, the cephalo-thorax is seen to be 

 slightly convex and divided into four portions by im- 

 perfect sutures. The frontal margin is indented. The 

 greatest depth is in the middle line. In the centre of 

 the hollow, situated on the ventral surface, is an oval- 

 shaped opening with a chitinous fringe. This opening 

 is apparently a rudimentary sucker, and is the remains 

 of a median sucker which is greatly developed in the 

 early life of the parasite. This rudimentary sucker can 

 be traced in all the members of the Caligoida. No 

 other suckers are present in the members of the genus 

 Lepeophtheirus. The frontal and lateral margins of 

 the segment are surrounded by a transparent mem- 

 brane with faint transverse lines. This membrane is 

 simply an extension of the chitinous exoskeleton 

 which covers the whole animal. The edges have fre- 

 quently a serrated appearance, but this is due to the 

 membrane being damaged. The eyes appear as a 

 reddish spot in the living animal. They are placed on 

 the dorsal surface midway between the anterior and 



