SEA-FISHERIES LABOKATOBX 203 



height of which varies considerably. In the anterior por- 

 tion of the stomach these folds are very little higher than 

 the general line, but as they pass posteriorly they increase 

 considerably, diminishing again in the intestine as they 

 approach the rectum. The greatest height of the folds 

 is reached in the portion of the intestine passing through 

 the genital segment. In the intestine and posterior por- 

 tion of the stomach there are a number of glandular cells, 

 usually at the apices of the longitudinal folds, the contents 

 of which stain deeply with eosin. In many of these the 

 cell contents have disappeared, leaving a clear space, 

 only the cell wall remaining. 



The wall of the stomach and intestine is marked by a 

 series of transverse constrictions, giving it a crenate 

 appearance, which is easily seen in the living animal. In 

 the living animal an intermittent movement of the intes- 

 tine and stomach is kept up. The action is wave-like, 

 starting at one end, and passing to the other. After con- 

 tinuing in one direction for a time, it reverses and passes 

 the opposite way. There is no valve between the stomach 

 and intestine, and when the peristaltic motion is reversed 

 the fluid in the intestine is sent back into the stomach 

 again. The only portions of the alimentary canal that 

 can be closed are the oesophagus and anal end of the 

 rectum. The former is controlled by two longitudinal 

 muscles which compress it, the latter by a number of 

 muscles passing obliquely to the body-wall at the sides of 

 the abdomen. The fluids contained in the alimentary 

 canal are usually colourless, but occasionally when taken 

 direct from the fish and placed under the microscope, a 

 reddish tint may be detected at the posterior end of the 

 oesophagus. 



In connection with the alimentary canal there is a 

 distinct paired digestive gland (Plate II., fig. o and fig. 9). 



