( 13 ) 



VI. — LIVER. 



(See Plate III.) 



The large gastric gland commonly called " liver " in Mollusca is of course well 

 known to be neither homologous nor strictly analogous with the liver of vertebrates. As 

 the sole digestive gland acting upon all constituents of the food, it is more nearly analogous 

 with the pancreas of higher animals, and has sometimes been called a hepato-pancreatic 

 gland. We are, however, familiar in some of the higher invertebrates with glandular caeca 

 of the mesenteron which act as stomach diverticula, and apparently receive, digest, and 

 absorb a portion of the food. This may well be the case to some extent with the liver 

 of the oyster. It may possibly have an absorbing as well as a secreting function to 

 perform in connection with digestion, and it is highly probable that, in addition, it has 

 important metabolic functions in connection with the elaboration and storing up of 

 reserve materials, including pigments, from the food. 



The liver communicates with the stomach by means of a wide passage on each 

 side (PI. III., Figs. 4 and 5), which almost at once divides (Fig. 13) into several 

 branches or hepatic ducts leading to the caecal lobules, which are lined by the hepatic 

 cells. The ducts, as was pointed out by Ryder in 1884, are lined by ciliated epithelium 

 resembling that of the stomach (compare Figs. 14, 15, 16, &c), and so can always be 

 easily distinguished in sections from the caeca, the walls of which are formed by the 

 large irregular hepatic cells (Figs. 6, 7, 8, &c). 



There is a considerable difference in the size of the liver and in the number of 

 lobules or caeca in different oysters, and, moreover, the size of the caeca and of the 

 hepatic cells seems to vary with the condition of nutrition of the animal, and also 

 becomes reduced at the breeding time. One of the first points we noticed in con- 

 nection with the green leucocytosis of certain American oysters, was, when examining 

 the animals fresh, that they seemed to us to have the liver in a shrunken and degenerate 

 condition. Carazzi objects to our use of this phrase, and seems to think that the liver 

 is always in the same condition. Even a superficial examination of a large number 

 of oysters, at different times and from different localities, is sufficient to show that he is 

 mistaken, and that the " good conditioned " appearance of the oyster depends largely 

 upon the state of the liver, as well as of the vesicular connective tissue around it. 

 We now show by our figures the comparison between the healthy (Fig. 10) and the 

 unhealthy (Fig. 11) liver, the latter showing the condition which we meant to indicate 

 by the terms " shrunken and degenerate " in our preliminary report. 



The colour also of the liver varies greatly, and may be dark brown (Figs. 1 

 and 2), bright yellow brown (Fig. 3), or dull green. The liver cells stain readily with 

 Ehrlich's hematoxylin, and with osmic acid granules can be demonstrated in abundance 



