37 



surface of the liver (fig. 17), and the two organs are sur- 

 rounded by a common iridescent membranous envelope, 

 outside and in addition to their individual coverings. 

 In a freshly obtained Eledone a bilobed oval whitish 

 region can be distinguished round the origin of the two 

 hepatic ducts (PI. VIII, fig. 32, P.). This is the so-called 

 pancreas, and shows up distinctly against the yellowish 

 green liver. The digestive gland as a whole is soft and 

 spongy, and enclosed in a very delicate membranous 

 envelope. It is built up of branching secretory tubules 

 which open into the hepatic ducts. The pancreatic 

 tubules likewise open into these ducts, further down. 



According to Bourquelot, the digestive hepato- 

 pancreatic fluid poured into the spiral caecum is 

 colourless before digestion, and brownish after it. The 

 hepatic secretion consists of diastase, trypsin and pepsin, 

 while the pancreas secretes diastase also. 



The opaque rather thick-walled hepatic ducts run 

 posteriorly, and after embracing the intestine unite to a 

 common channel which enters into the spiral caecum 

 (fig. 28). Hence the order of events in the digestive 

 economy of Eledone is as follows : — 



(1) Food seized by the suckers is torn up by the jaws 

 and passed into the mouth. 



(2) Here it is mixed with the mucous secretion of the 

 sub-lingual and posterior salivary glands. 



(3) Next the radula rasps it and further breaks it up. 



(4) As it passes into the oesophagus the secretion of 

 the anterior salivary glands is poured over it. 



(5) Now it passes to the stomach. Here the food is 

 ground and mixed well. The hepato-pancreatic ferments 

 enter from the caecum or reservoir, and become mixed 

 with the food, and so digestion takes place. 



(6) Next the food passes on out of the stomach into 



