214 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



which, has been long preserved the components of the visceral 

 mass are extremely difficult to separate. In the fresh con- 

 dition, however, this is possible. The outline of the liver is 

 smooth and rounded. It shows no trace of spiral coiling, but 

 is divided into irregular lobes. 



Microscopically the liver (Fig. 21) consists of simple 

 groups of cells lining central spaces, the spaces uniting to form 

 ciliated channels, the bile ducts. The latter are of wide 

 calibre and discharge by four orifices into the stomach. The 

 hepatic cells contain dark brown or greenish granular pigment 

 and frequently drops of oil. The bile fluid is in part digestive 

 and in part excretory. 



Mechanism of Digestion. 



The food, which consists of seaweeds of various species, 

 is grasped by the protruding lips and jaws and rasped off in 

 small pieces by the action of the radula. To do this the whole 

 odontophore is thrust forwards so that the radular ribbon 

 works against the flat surface of the jaws. The backwardly- 

 directed teeth of the radula scrape off particles of the weed, 

 and the mucous secretion of the salivary glands mixes with 

 the food and moistens it. The food is then stored in the 

 crop. Relaxation of the sphincter muscle between the crop 

 and the gizzard allows the food to pass into the latter in small 

 quantities at a time. Here the horny gizzard teeth squeeze 

 and strain the fragments. The rate of entrance into the 

 stomach proper is regulated by means of the strong flap-like 

 valves which guard the aperture. In this chamber the digestive 

 juice from the liver is poured on the food, which then travels 

 slowly down one side of the caecum, up the other side, and so 

 into the intestine. By this time digestion is practically 

 complete, the digested matter is absorbed, and the waste 

 products, in the form of loose irregular pellets, are expelled 

 through the anus. 



