12 BRITISH PARASITIC COPEl'ODA. 



COLOUR. 



The colour of the living animals varies with the 

 position in which they live. On the dark side of the 

 fish they are of a deep brown, almost black, colour. On 

 the "white " side and under the fins they are nearly 

 colourless, due to the contraction of the pigment-cells, 

 which appear as brown spots under the microscope. 

 The dark-coloured specimens soon become colourless 

 when exposed to light. 



ALIMENTARY SYSTEM, ETC. 



The body- wall consists of (1) chitinous cuticle or 

 exoskeleton, (2) cellular hypodermis, and (3) connective 

 tissue laminae which line the integument, traverse the 

 body-cavity, and support the alimentary canal and 

 other organs. The only cavity left inside the body- 

 wall is the system of lacunae in which the colourless 

 blood flows. 



The mouth, already described, leads into a short 

 narrow oesophagus, lined with a thin chitinous coat 

 which is continuous with the exoskeleton. The oeso- 

 phagus passes through the anterior part of the nervous 

 system, and in a transverse section of that region 

 appears as a minute pinhole. The stomach lies along 

 the ventral surface and is lageniform in shape. At 

 the anterior end it is produced into a short caecum, 

 \vhich extends over the posterior end of the oesophagus 

 and terminates by opening into the intestine. The 

 intestine is simply a direct continuation of the stomach 

 and terminates in a short rectum leading into the anus 

 at the apex of the abdomen. There are no convolu- 

 tions in the alimentary canal. The wall of the whole 

 alimentary canal is lined with a thin layer of chitin 



