472 FUNCTION OF THE MID-GUT GLAND OF MOLLUSCA. [BOOK II. 



parasitic animals belonging to the sub-kingdoms above Protozoa 

 possess a differentiated digestive function, and, with few exceptions, 

 have been found to form digestive enzymes. 



According to Krukenberg 1 , the Hydro-medusae and the sea- 

 Anemones, although possessed of differentiated alimentary apparatus, 

 form no secretion possessed of digestive properties. The external 

 slimy secretion of certain of these creatures in some cases possesses 

 active irritating properties, and appears to subserve defensive purposes,, 

 but is destitute of digestive enzymes. 



SECT 2. THE FUNCTION OF THE SO-CALLED 

 MOLLUSCA. 



LIVER' OF 



The function In connection with the mid-gut of the Mollusca there 



of the 'mid-gut is developed, by the sacculation of the endoderm, a 

 gland' or liver tabulated glandular organ, which opens into the enteron. 

 This organ, which has usually been termed by compa- 

 rative anatomists the liver (sometimes the ' hepato-pancreas '), and 

 which appears to be the morphological homologue of the organ 

 bearing that name in Vertebrata, discharges functions which are 

 altogether unlike those of the liver. It is destitute of either of the 

 specific products bile-colouring matters and bile-acids which are 

 characteristic of the secretory activity of the hepatic cells ; but con- 

 tains several enzymes which confer upon it powers of digesting the 

 various groups of alimentary principles. 



From the researches of Krukenberg 2 it results that this gland 

 secretes a diastatic, a fat-splitting and either a pepsin-like or a 

 trypsin-like enzyme. In some molluscs as in Helix pomatia, which 

 possess an acid intestinal digestion, the mid-gut gland forms a ferment 

 like pepsin but secretes no trypsin, whilst in other molluscs no pepsin 

 is present, but trypsin. A. B. Griffiths 3 has compared the secretion of 

 the 'liver ' of Cephalopoda with that of the pancreas, and has obtained 

 results which prove the identity of function in so far as the creatures he 

 investigated were concerned. In Cephalopoda the secretion is alka- 

 line, converts starch into sugar, emulsifies fats, and dissolves proteids* 



Though not discharging the functions of the liver as a bile- 

 secreting gland, it is conceivable that the mid-gut gland of Mollusca 

 might fulfil another function which the liver of vertebrates pos- 

 sesses that of a storer of reserve material, of a former and hoarder 

 of glycogen. The painstaking researches of Levy 4 have shown, how- 

 ever, that although glycogen is a constituent of the gland, it is 



1 C. Fr. W. Krukenberg, Grundziige einer vergleichenden Physiologie der Verdauung. 

 Heidelberg (Carl Winter), 1882. 



2 Krukenberg, Grundziige, etc. pp. 59 and 61. 



3 A. B. Griffiths, Chemisch-physiologische Untersuchungen iiber die Cephaloden- 

 Leber und ihre Identitat mit einem wahren Pankreas,' Ber. d. deutsch. chem. Gesellsch. 

 Vol. xviii. (1885), p. 294. This is a short abstract of the original paper in the 

 Chemical News, Vol. LI. p. 160, which the Author has not had access to. 



4 Max Levy, * Zoochemische Untersuchungen der Mitteldarmdriise von Helix 

 pomatia,' Zeitsch. f. Biologie, Vol. xxv. (1890), p. 410. 



