76 
The colour of the living animal varies with the position 
in which it lives. 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 
forms soon become almost colourless when exposed to 
light. 
Tuer Bopy-watLt AND Bopy-caVITY. 
The body-wall consists of (1) the chitinous cuticle or 
exoskeleton, which has been described in the external 
characters, (2) the cellular hvpodermis, and (3) the con- 
nective tissue laminew 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 lacune, in which the colourless blood flows 
(see below under blood system, p. 81). 
THE ALIMENTARY CANAL. 
The mouth, already described, leads into a short, narrow 
curved oesophagus, lined with a thin chitinous coat which 
is continuous with the exoskeleton. Near the anterior 
end the chitinous coat is much folded. The cesophagus 
(Plate ITT., figs. 5 and 5) passes through the anterior part 
of the nervous system, and in a transverse section of that 
region appears as a minute pinhole. After leaving the 
nervous system, it courses over the sub-cesophageal gang- 
lion, and under a short cecal projection of the stomach, 
finally entering the stomach on its ventral aspect, at the 
posterior end of the sub-cesophageal ganglion. 
The stomach lies along the ventral surface, and is 
lageniform in shape (Plate IT., fig. 3). At the anterior 
end it is produced into a short cecum which extends over 
the posterior end of the esophagus and it terminates by 
