SEA-FISHERIES LABORATORY. 201 



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. 



The Body-wall and Body-cavity. 



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

 exoskeleton, which has been described in the external 

 characters, (2) the cellular hypodermis, and (3) the con- 

 nective 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 

 (see below under blood system, p. 20). 



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 oesophagus 

 (Plate III., figs. 3 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-oesophageal gang- 

 lion, and under a short caecal projection of the stomach, 

 finally entering the stomach on its ventral aspect, at the 

 posterior end of the sub-oesophageal ganglion. 



The stomach lies along the ventral surface, and is 

 lageniform in shape (Plate II., fig. 3). At the anterior 

 end it is produced into a short caecum which extends over 

 the posterior end of the oesophagus and it terminates by 



