INTRODUCTION. 13 



continuous with the exterior. The wall of the stomach 

 and intestines is marked by a series of transverse con- 

 strictions, giving it a crenate appearance which is 

 easily seen in the living animal. When the animal is 

 alive an intermittent movement of the alimentary canal 

 is maintained. The action is wave-like. It begins at 

 one end and passes slowly to the other. After con- 

 tinuing in one direction for a time, it reverses and 

 passes along the opposite way. The fluids in the 

 alimentary canal are usually colourless. Sometimes, 

 however, when taken direct from the fish and placed 

 under the microscope, a reddish tint may be observed 

 at the posterior end of the oesophagus. Two pairs of 

 digestive glands are connected with the alimentary 

 canal. The first and most important pair is situated 

 in the anterior end of the cephalo-thorax. Each gland 

 consists of three portions, two moderately large masses 

 on the lateral margins just behind the antennules, and 

 a median, smaller one, in front of the base of the 

 mouth. The lateral masses are connected with the 

 median one by a duct. A duct is given off by the 

 median portion and passes posteriorly along the out- 

 side of the oesophagus. It enters the caecum at the 

 anterior end of the stomach. "When the parasite is 

 first removed from the host the glands are usually of 

 a dark brown colour, but they soon become bleached. 

 The product of the glands is a pale yellow fluid. The 

 second pair of glands is much smaller than the first, 

 and is situated close to the median line between the 

 first and second pairs of feet. The glands are of 

 a brown colour and shaped like a comma. A minute 

 duct from each gland passes downward and then 

 forward along the stomach, which it enters near the 

 posterior end. 



