1893.] and Functions of the Alimentary Canal o/Daphnia. 43 



We have not succeeded however in determining how this is 

 done. 



When the bolus is complete it is rapidly carried into the 

 niesenteron by a peristaltic movement of the oesophagus, which 

 in its resting condition is a closed tube flattened dorso-ven- 

 trally. The peristaltic movement spoken of above consists of a 

 wave of dilatation which starts at the mouth, and which is fol- 

 lowed by a wave of constriction. The bolus is sucked into the 

 oesophagus by the first wave and then thrown with a certain 

 degree of force into the mesenteron. 



When the peristaltic wave reaches the mesenteron it is con- 

 tinued backwards over that structure and carries the bolus before 

 it as far as the middle third. 



The act of deglutition therefore is brought about by a peri- 

 staltic wave which starts at the mouth as a result of the stimu- 

 lation of the sensory surfaces there and runs backwards carrying 

 the food along the gut to the middle third or digestive region 

 of the mesenteron. 



Digestion. The food when swallowed consists of particles, 

 e.g. precipitated proteid, fat globules, carmine grains, or uni- 

 cellular plants, which are glued together by some sticky substance. 

 As digestion proceeds the insoluble particles are slowly liberated 

 and some are carried into the anterior region of the midgut and 

 even into the liver diverticula. There the nutritive particles 

 such as the fat globules of milk or yolk (but not the carmine 

 grains) are ingested by the columnar cells lining that region. 

 If the food contain any soluble colouring matter, such for in- 

 stance as chlorophyll, that also accumulates in the anterior region. 

 The non-nutritive particles do not accumulate in the anterior 

 region but are from time to time driven backwards. Thus, in 

 the case of an animal which has been fed on green algae we 

 find the anterior third of the mesenteron including the liver 

 diverticula occupied with a bright-green fluid in which are sus- 

 pended a few solid particles; the middle region occupied by a 

 dark-green mass of still undigested food, while the posterior 

 region contains a mass of brown particles which will form the faeces. 



The movements of the gut which bring about this distribution 

 of the contents are twofold. First there is a constantly occurring 

 peristalsis which consists of waves occurring at regular intervals. 

 These start at the junction of mesenteron and proctodseum and 

 run forwards, ultimately dying away in the liver diverticula. 

 They are best developed, that is they lead to the most con- 

 siderable constriction, in the posterior region, and they are 

 apparently conditioned by the presence of food particles or of 

 their soluble products in the lumen of the gut. They, however, 

 are sometimes seen in starving animals. 



4—2 



